diff --git a/src/db.py b/src/db.py
index 64aeac7..0a43424 100644
--- a/src/db.py
+++ b/src/db.py
@@ -55,6 +55,28 @@ def init_db(db_path: str) -> sqlite3.Connection:
         """
     )
 
+    # Decision logging table for comprehensive audit trail
+    conn.execute(
+        """
+        CREATE TABLE IF NOT EXISTS decision_logs (
+            decision_id TEXT PRIMARY KEY,
+            timestamp TEXT NOT NULL,
+            stock_code TEXT NOT NULL,
+            market TEXT NOT NULL,
+            exchange_code TEXT NOT NULL,
+            action TEXT NOT NULL,
+            confidence INTEGER NOT NULL,
+            rationale TEXT NOT NULL,
+            context_snapshot TEXT NOT NULL,
+            input_data TEXT NOT NULL,
+            outcome_pnl REAL,
+            outcome_accuracy INTEGER,
+            reviewed INTEGER DEFAULT 0,
+            review_notes TEXT
+        )
+        """
+    )
+
     conn.execute(
         """
         CREATE TABLE IF NOT EXISTS context_metadata (
@@ -71,6 +93,16 @@ def init_db(db_path: str) -> sqlite3.Connection:
     conn.execute("CREATE INDEX IF NOT EXISTS idx_contexts_timeframe ON contexts(timeframe)")
     conn.execute("CREATE INDEX IF NOT EXISTS idx_contexts_updated ON contexts(updated_at)")
 
+    # Create indices for efficient decision log queries
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_decision_logs_timestamp ON decision_logs(timestamp)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_decision_logs_reviewed ON decision_logs(reviewed)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_decision_logs_confidence ON decision_logs(confidence)"
+    )
     conn.commit()
     return conn
 
diff --git a/src/logging/__init__.py b/src/logging/__init__.py
new file mode 100644
index 0000000..6ad189a
--- /dev/null
+++ b/src/logging/__init__.py
@@ -0,0 +1,5 @@
+"""Decision logging and audit trail for trade decisions."""
+
+from src.logging.decision_logger import DecisionLog, DecisionLogger
+
+__all__ = ["DecisionLog", "DecisionLogger"]
diff --git a/src/logging/decision_logger.py b/src/logging/decision_logger.py
new file mode 100644
index 0000000..b2f52a5
--- /dev/null
+++ b/src/logging/decision_logger.py
@@ -0,0 +1,235 @@
+"""Decision logging system with context snapshots for comprehensive audit trail."""
+
+from __future__ import annotations
+
+import json
+import sqlite3
+import uuid
+from dataclasses import dataclass
+from datetime import UTC, datetime
+from typing import Any
+
+
+@dataclass
+class DecisionLog:
+    """A logged trading decision with context and outcome."""
+
+    decision_id: str
+    timestamp: str
+    stock_code: str
+    market: str
+    exchange_code: str
+    action: str
+    confidence: int
+    rationale: str
+    context_snapshot: dict[str, Any]
+    input_data: dict[str, Any]
+    outcome_pnl: float | None = None
+    outcome_accuracy: int | None = None
+    reviewed: bool = False
+    review_notes: str | None = None
+
+
+class DecisionLogger:
+    """Logs trading decisions with full context for review and evolution."""
+
+    def __init__(self, conn: sqlite3.Connection) -> None:
+        """Initialize the decision logger with a database connection."""
+        self.conn = conn
+
+    def log_decision(
+        self,
+        stock_code: str,
+        market: str,
+        exchange_code: str,
+        action: str,
+        confidence: int,
+        rationale: str,
+        context_snapshot: dict[str, Any],
+        input_data: dict[str, Any],
+    ) -> str:
+        """Log a trading decision with full context.
+
+        Args:
+            stock_code: Stock symbol
+            market: Market code (e.g., "KR", "US_NASDAQ")
+            exchange_code: Exchange code (e.g., "KRX", "NASDAQ")
+            action: Trading action (BUY/SELL/HOLD)
+            confidence: Confidence level (0-100)
+            rationale: Reasoning for the decision
+            context_snapshot: L1-L7 context snapshot at decision time
+            input_data: Market data inputs (price, volume, orderbook, etc.)
+
+        Returns:
+            decision_id: Unique identifier for this decision
+        """
+        decision_id = str(uuid.uuid4())
+        timestamp = datetime.now(UTC).isoformat()
+
+        self.conn.execute(
+            """
+            INSERT INTO decision_logs (
+                decision_id, timestamp, stock_code, market, exchange_code,
+                action, confidence, rationale, context_snapshot, input_data
+            )
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+            """,
+            (
+                decision_id,
+                timestamp,
+                stock_code,
+                market,
+                exchange_code,
+                action,
+                confidence,
+                rationale,
+                json.dumps(context_snapshot),
+                json.dumps(input_data),
+            ),
+        )
+        self.conn.commit()
+
+        return decision_id
+
+    def get_unreviewed_decisions(
+        self, min_confidence: int = 80, limit: int | None = None
+    ) -> list[DecisionLog]:
+        """Get unreviewed decisions with high confidence.
+
+        Args:
+            min_confidence: Minimum confidence threshold (default 80)
+            limit: Maximum number of results (None = unlimited)
+
+        Returns:
+            List of unreviewed DecisionLog objects
+        """
+        query = """
+            SELECT
+                decision_id, timestamp, stock_code, market, exchange_code,
+                action, confidence, rationale, context_snapshot, input_data,
+                outcome_pnl, outcome_accuracy, reviewed, review_notes
+            FROM decision_logs
+            WHERE reviewed = 0 AND confidence >= ?
+            ORDER BY timestamp DESC
+        """
+        if limit is not None:
+            query += f" LIMIT {limit}"
+
+        cursor = self.conn.execute(query, (min_confidence,))
+        return [self._row_to_decision_log(row) for row in cursor.fetchall()]
+
+    def mark_reviewed(self, decision_id: str, notes: str) -> None:
+        """Mark a decision as reviewed with notes.
+
+        Args:
+            decision_id: Decision identifier
+            notes: Review notes and insights
+        """
+        self.conn.execute(
+            """
+            UPDATE decision_logs
+            SET reviewed = 1, review_notes = ?
+            WHERE decision_id = ?
+            """,
+            (notes, decision_id),
+        )
+        self.conn.commit()
+
+    def update_outcome(
+        self, decision_id: str, pnl: float, accuracy: int
+    ) -> None:
+        """Update the outcome of a decision after trade execution.
+
+        Args:
+            decision_id: Decision identifier
+            pnl: Actual profit/loss realized
+            accuracy: 1 if decision was correct, 0 if wrong
+        """
+        self.conn.execute(
+            """
+            UPDATE decision_logs
+            SET outcome_pnl = ?, outcome_accuracy = ?
+            WHERE decision_id = ?
+            """,
+            (pnl, accuracy, decision_id),
+        )
+        self.conn.commit()
+
+    def get_decision_by_id(self, decision_id: str) -> DecisionLog | None:
+        """Get a specific decision by ID.
+
+        Args:
+            decision_id: Decision identifier
+
+        Returns:
+            DecisionLog object or None if not found
+        """
+        cursor = self.conn.execute(
+            """
+            SELECT
+                decision_id, timestamp, stock_code, market, exchange_code,
+                action, confidence, rationale, context_snapshot, input_data,
+                outcome_pnl, outcome_accuracy, reviewed, review_notes
+            FROM decision_logs
+            WHERE decision_id = ?
+            """,
+            (decision_id,),
+        )
+        row = cursor.fetchone()
+        return self._row_to_decision_log(row) if row else None
+
+    def get_losing_decisions(
+        self, min_confidence: int = 80, min_loss: float = -100.0
+    ) -> list[DecisionLog]:
+        """Get high-confidence decisions that resulted in losses.
+
+        Useful for identifying patterns in failed predictions.
+
+        Args:
+            min_confidence: Minimum confidence threshold (default 80)
+            min_loss: Minimum loss amount (default -100.0, i.e., loss >= 100)
+
+        Returns:
+            List of losing DecisionLog objects
+        """
+        cursor = self.conn.execute(
+            """
+            SELECT
+                decision_id, timestamp, stock_code, market, exchange_code,
+                action, confidence, rationale, context_snapshot, input_data,
+                outcome_pnl, outcome_accuracy, reviewed, review_notes
+            FROM decision_logs
+            WHERE confidence >= ?
+              AND outcome_pnl IS NOT NULL
+              AND outcome_pnl <= ?
+            ORDER BY outcome_pnl ASC
+            """,
+            (min_confidence, min_loss),
+        )
+        return [self._row_to_decision_log(row) for row in cursor.fetchall()]
+
+    def _row_to_decision_log(self, row: tuple[Any, ...]) -> DecisionLog:
+        """Convert a database row to a DecisionLog object.
+
+        Args:
+            row: Database row tuple
+
+        Returns:
+            DecisionLog object
+        """
+        return DecisionLog(
+            decision_id=row[0],
+            timestamp=row[1],
+            stock_code=row[2],
+            market=row[3],
+            exchange_code=row[4],
+            action=row[5],
+            confidence=row[6],
+            rationale=row[7],
+            context_snapshot=json.loads(row[8]),
+            input_data=json.loads(row[9]),
+            outcome_pnl=row[10],
+            outcome_accuracy=row[11],
+            reviewed=bool(row[12]),
+            review_notes=row[13],
+        )
diff --git a/src/main.py b/src/main.py
index c95d86c..a5ae929 100644
--- a/src/main.py
+++ b/src/main.py
@@ -19,6 +19,7 @@ from src.broker.overseas import OverseasBroker
 from src.config import Settings
 from src.core.risk_manager import CircuitBreakerTripped, RiskManager
 from src.db import init_db, log_trade
+from src.logging.decision_logger import DecisionLogger
 from src.logging_config import setup_logging
 from src.markets.schedule import MarketInfo, get_next_market_open, get_open_markets
 
@@ -42,6 +43,7 @@ async def trading_cycle(
     brain: GeminiClient,
     risk: RiskManager,
     db_conn: Any,
+    decision_logger: DecisionLogger,
     market: MarketInfo,
     stock_code: str,
 ) -> None:
@@ -101,6 +103,39 @@ async def trading_cycle(
         decision.confidence,
     )
 
+    # 2.5. Log decision with context snapshot
+    context_snapshot = {
+        "L1": {
+            "current_price": current_price,
+            "foreigner_net": foreigner_net,
+        },
+        "L2": {
+            "total_eval": total_eval,
+            "total_cash": total_cash,
+            "purchase_total": purchase_total,
+            "pnl_pct": pnl_pct,
+        },
+        # L3-L7 will be populated when context tree is implemented
+    }
+    input_data = {
+        "current_price": current_price,
+        "foreigner_net": foreigner_net,
+        "total_eval": total_eval,
+        "total_cash": total_cash,
+        "pnl_pct": pnl_pct,
+    }
+
+    decision_logger.log_decision(
+        stock_code=stock_code,
+        market=market.code,
+        exchange_code=market.exchange_code,
+        action=decision.action,
+        confidence=decision.confidence,
+        rationale=decision.rationale,
+        context_snapshot=context_snapshot,
+        input_data=input_data,
+    )
+
     # 3. Execute if actionable
     if decision.action in ("BUY", "SELL"):
         # Determine order size (simplified: 1 lot)
@@ -151,6 +186,7 @@ async def run(settings: Settings) -> None:
     brain = GeminiClient(settings)
     risk = RiskManager(settings)
     db_conn = init_db(settings.DB_PATH)
+    decision_logger = DecisionLogger(db_conn)
 
     shutdown = asyncio.Event()
 
@@ -218,6 +254,7 @@ async def run(settings: Settings) -> None:
                                 brain,
                                 risk,
                                 db_conn,
+                                decision_logger,
                                 market,
                                 stock_code,
                             )
diff --git a/tests/test_decision_logger.py b/tests/test_decision_logger.py
new file mode 100644
index 0000000..652d3c3
--- /dev/null
+++ b/tests/test_decision_logger.py
@@ -0,0 +1,292 @@
+"""Tests for decision logging and audit trail."""
+
+from __future__ import annotations
+
+import sqlite3
+from datetime import UTC, datetime
+
+import pytest
+
+from src.db import init_db
+from src.logging.decision_logger import DecisionLog, DecisionLogger
+
+
+@pytest.fixture
+def db_conn() -> sqlite3.Connection:
+    """Provide an in-memory database with initialized schema."""
+    conn = init_db(":memory:")
+    return conn
+
+
+@pytest.fixture
+def logger(db_conn: sqlite3.Connection) -> DecisionLogger:
+    """Provide a DecisionLogger instance."""
+    return DecisionLogger(db_conn)
+
+
+def test_log_decision_creates_record(logger: DecisionLogger, db_conn: sqlite3.Connection) -> None:
+    """Test that log_decision creates a database record."""
+    context_snapshot = {
+        "L1": {"quote": {"price": 100.0, "volume": 1000}},
+        "L2": {"orderbook": {"bid": [99.0], "ask": [101.0]}},
+    }
+    input_data = {"price": 100.0, "volume": 1000, "foreigner_net": 500}
+
+    decision_id = logger.log_decision(
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=85,
+        rationale="Strong upward momentum",
+        context_snapshot=context_snapshot,
+        input_data=input_data,
+    )
+
+    # Verify decision_id is a valid UUID
+    assert decision_id is not None
+    assert len(decision_id) == 36  # UUID v4 format
+
+    # Verify record exists in database
+    cursor = db_conn.execute(
+        "SELECT decision_id, action, confidence FROM decision_logs WHERE decision_id = ?",
+        (decision_id,),
+    )
+    row = cursor.fetchone()
+    assert row is not None
+    assert row[0] == decision_id
+    assert row[1] == "BUY"
+    assert row[2] == 85
+
+
+def test_log_decision_stores_context_snapshot(logger: DecisionLogger) -> None:
+    """Test that context snapshot is stored as JSON."""
+    context_snapshot = {
+        "L1": {"real_time": "data"},
+        "L3": {"daily": "aggregate"},
+        "L7": {"legacy": "wisdom"},
+    }
+    input_data = {"price": 50000.0, "volume": 2000}
+
+    decision_id = logger.log_decision(
+        stock_code="035420",
+        market="KR",
+        exchange_code="KRX",
+        action="HOLD",
+        confidence=75,
+        rationale="Waiting for clearer signal",
+        context_snapshot=context_snapshot,
+        input_data=input_data,
+    )
+
+    # Retrieve and verify context snapshot
+    decision = logger.get_decision_by_id(decision_id)
+    assert decision is not None
+    assert decision.context_snapshot == context_snapshot
+    assert decision.input_data == input_data
+
+
+def test_get_unreviewed_decisions(logger: DecisionLogger) -> None:
+    """Test retrieving unreviewed decisions with confidence filter."""
+    # Log multiple decisions with varying confidence
+    logger.log_decision(
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=90,
+        rationale="High confidence buy",
+        context_snapshot={},
+        input_data={},
+    )
+    logger.log_decision(
+        stock_code="000660",
+        market="KR",
+        exchange_code="KRX",
+        action="SELL",
+        confidence=75,
+        rationale="Low confidence sell",
+        context_snapshot={},
+        input_data={},
+    )
+    logger.log_decision(
+        stock_code="035420",
+        market="KR",
+        exchange_code="KRX",
+        action="HOLD",
+        confidence=85,
+        rationale="Medium confidence hold",
+        context_snapshot={},
+        input_data={},
+    )
+
+    # Get unreviewed decisions with default threshold (80)
+    unreviewed = logger.get_unreviewed_decisions()
+    assert len(unreviewed) == 2  # Only confidence >= 80
+    assert all(d.confidence >= 80 for d in unreviewed)
+    assert all(not d.reviewed for d in unreviewed)
+
+    # Get with lower threshold
+    unreviewed_all = logger.get_unreviewed_decisions(min_confidence=70)
+    assert len(unreviewed_all) == 3
+
+
+def test_mark_reviewed(logger: DecisionLogger) -> None:
+    """Test marking a decision as reviewed."""
+    decision_id = logger.log_decision(
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=85,
+        rationale="Test decision",
+        context_snapshot={},
+        input_data={},
+    )
+
+    # Initially unreviewed
+    decision = logger.get_decision_by_id(decision_id)
+    assert decision is not None
+    assert not decision.reviewed
+    assert decision.review_notes is None
+
+    # Mark as reviewed
+    review_notes = "Good decision, captured bullish momentum correctly"
+    logger.mark_reviewed(decision_id, review_notes)
+
+    # Verify updated
+    decision = logger.get_decision_by_id(decision_id)
+    assert decision is not None
+    assert decision.reviewed
+    assert decision.review_notes == review_notes
+
+    # Should not appear in unreviewed list
+    unreviewed = logger.get_unreviewed_decisions()
+    assert all(d.decision_id != decision_id for d in unreviewed)
+
+
+def test_update_outcome(logger: DecisionLogger) -> None:
+    """Test updating decision outcome with P&L and accuracy."""
+    decision_id = logger.log_decision(
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=90,
+        rationale="Expecting price increase",
+        context_snapshot={},
+        input_data={},
+    )
+
+    # Initially no outcome
+    decision = logger.get_decision_by_id(decision_id)
+    assert decision is not None
+    assert decision.outcome_pnl is None
+    assert decision.outcome_accuracy is None
+
+    # Update outcome (profitable trade)
+    logger.update_outcome(decision_id, pnl=5000.0, accuracy=1)
+
+    # Verify updated
+    decision = logger.get_decision_by_id(decision_id)
+    assert decision is not None
+    assert decision.outcome_pnl == 5000.0
+    assert decision.outcome_accuracy == 1
+
+
+def test_get_losing_decisions(logger: DecisionLogger) -> None:
+    """Test retrieving high-confidence losing decisions."""
+    # Profitable decision
+    id1 = logger.log_decision(
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=85,
+        rationale="Correct prediction",
+        context_snapshot={},
+        input_data={},
+    )
+    logger.update_outcome(id1, pnl=3000.0, accuracy=1)
+
+    # High-confidence loss
+    id2 = logger.log_decision(
+        stock_code="000660",
+        market="KR",
+        exchange_code="KRX",
+        action="SELL",
+        confidence=90,
+        rationale="Wrong prediction",
+        context_snapshot={},
+        input_data={},
+    )
+    logger.update_outcome(id2, pnl=-2000.0, accuracy=0)
+
+    # Low-confidence loss (should be ignored)
+    id3 = logger.log_decision(
+        stock_code="035420",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=70,
+        rationale="Low confidence, wrong",
+        context_snapshot={},
+        input_data={},
+    )
+    logger.update_outcome(id3, pnl=-1500.0, accuracy=0)
+
+    # Get high-confidence losing decisions
+    losers = logger.get_losing_decisions(min_confidence=80, min_loss=-1000.0)
+    assert len(losers) == 1
+    assert losers[0].decision_id == id2
+    assert losers[0].outcome_pnl == -2000.0
+    assert losers[0].confidence == 90
+
+
+def test_get_decision_by_id_not_found(logger: DecisionLogger) -> None:
+    """Test that get_decision_by_id returns None for non-existent ID."""
+    decision = logger.get_decision_by_id("non-existent-uuid")
+    assert decision is None
+
+
+def test_unreviewed_limit(logger: DecisionLogger) -> None:
+    """Test that get_unreviewed_decisions respects limit parameter."""
+    # Create 5 unreviewed decisions
+    for i in range(5):
+        logger.log_decision(
+            stock_code=f"00{i}",
+            market="KR",
+            exchange_code="KRX",
+            action="HOLD",
+            confidence=85,
+            rationale=f"Decision {i}",
+            context_snapshot={},
+            input_data={},
+        )
+
+    # Get only 3
+    unreviewed = logger.get_unreviewed_decisions(limit=3)
+    assert len(unreviewed) == 3
+
+
+def test_decision_log_dataclass() -> None:
+    """Test DecisionLog dataclass creation."""
+    now = datetime.now(UTC).isoformat()
+    log = DecisionLog(
+        decision_id="test-uuid",
+        timestamp=now,
+        stock_code="005930",
+        market="KR",
+        exchange_code="KRX",
+        action="BUY",
+        confidence=85,
+        rationale="Test",
+        context_snapshot={"L1": "data"},
+        input_data={"price": 100.0},
+    )
+
+    assert log.decision_id == "test-uuid"
+    assert log.action == "BUY"
+    assert log.confidence == 85
+    assert log.reviewed is False
+    assert log.outcome_pnl is None