diff --git a/src/services/categorizer.py b/src/services/categorizer.py
new file mode 100644
index 0000000..6db8410
--- /dev/null
+++ b/src/services/categorizer.py
@@ -0,0 +1,57 @@
+import re
+from dataclasses import dataclass
+from typing import Protocol, runtime_checkable
+
+from sqlalchemy.orm import Session
+
+from src.models.rule import CategorizationRule
+from src.models.transaction import Transaction
+
+
+@dataclass
+class CategoryResult:
+    category_id: int
+    tag: str | None = None
+    attributed_to_id: int | None = None
+
+
+@runtime_checkable
+class Categorizer(Protocol):
+    def categorize(self, transaction: Transaction) -> CategoryResult | None: ...
+
+
+class RuleBasedCategorizer:
+    def __init__(self, session: Session):
+        self.session = session
+        self._rules: list[CategorizationRule] | None = None
+
+    def _load_rules(self) -> list[CategorizationRule]:
+        if self._rules is None:
+            self._rules = (
+                self.session.query(CategorizationRule)
+                .order_by(CategorizationRule.priority)
+                .all()
+            )
+        return self._rules
+
+    def invalidate_cache(self) -> None:
+        self._rules = None
+
+    def categorize(self, transaction: Transaction) -> CategoryResult | None:
+        rules = self._load_rules()
+        desc = transaction.description.upper()
+
+        for rule in rules:
+            patterns = [p.strip() for p in rule.pattern.split("|")]
+            for pattern in patterns:
+                if re.search(re.escape(pattern.upper()), desc):
+                    tag = rule.tag_override
+                    if tag is None and rule.category:
+                        tag = rule.category.default_tag
+                    return CategoryResult(
+                        category_id=rule.category_id,
+                        tag=tag,
+                        attributed_to_id=rule.attributed_to_id,
+                    )
+
+        return None
diff --git a/tests/services/test_categorizer.py b/tests/services/test_categorizer.py
new file mode 100644
index 0000000..8e14f92
--- /dev/null
+++ b/tests/services/test_categorizer.py
@@ -0,0 +1,110 @@
+import datetime
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+
+from src.db import Base
+from src.models import *
+from src.seed import seed_categories
+from src.services.categorizer import RuleBasedCategorizer, Categorizer
+
+
+def make_session():
+    engine = create_engine("sqlite:///:memory:")
+    Base.metadata.create_all(engine)
+    return Session(engine)
+
+
+def test_categorizer_protocol():
+    """RuleBasedCategorizer implements Categorizer protocol."""
+    session = make_session()
+    cat = RuleBasedCategorizer(session)
+    assert isinstance(cat, Categorizer)
+
+
+def test_match_simple_pattern():
+    session = make_session()
+    seed_categories(session)
+    groceries = session.query(Category).filter_by(name="Groceries").one()
+    rule = CategorizationRule(pattern="PUBLIX", category_id=groceries.id, priority=10)
+    session.add(rule)
+    session.commit()
+
+    cat = RuleBasedCategorizer(session)
+    txn = Transaction(date=datetime.date(2026, 1, 15), amount=-44.90, description="PUBLIX #1716", account_id=1)
+    result = cat.categorize(txn)
+    assert result is not None
+    assert result.category_id == groceries.id
+
+
+def test_match_pipe_separated_pattern():
+    session = make_session()
+    seed_categories(session)
+    groceries = session.query(Category).filter_by(name="Groceries").one()
+    rule = CategorizationRule(pattern="PUBLIX|ALDI|PIGGLY WIGGLY", category_id=groceries.id, priority=10)
+    session.add(rule)
+    session.commit()
+
+    cat = RuleBasedCategorizer(session)
+    for desc in ["PUBLIX #1716", "ALDI 76180  BEAUFORT", "PIGGLY WIGGLY #286"]:
+        txn = Transaction(date=datetime.date(2026, 1, 15), amount=-20.00, description=desc, account_id=1)
+        result = cat.categorize(txn)
+        assert result is not None, f"Failed to match: {desc}"
+        assert result.category_id == groceries.id
+
+
+def test_no_match_returns_none():
+    session = make_session()
+    seed_categories(session)
+    cat = RuleBasedCategorizer(session)
+    txn = Transaction(date=datetime.date(2026, 1, 15), amount=-10.00, description="UNKNOWN MERCHANT", account_id=1)
+    result = cat.categorize(txn)
+    assert result is None
+
+
+def test_priority_ordering():
+    session = make_session()
+    seed_categories(session)
+    groceries = session.query(Category).filter_by(name="Groceries").one()
+    shopping = session.query(Category).filter_by(name="Shopping").one()
+    rule1 = CategorizationRule(pattern="WAL-MART", category_id=groceries.id, priority=1)
+    rule2 = CategorizationRule(pattern="WAL", category_id=shopping.id, priority=10)
+    session.add_all([rule1, rule2])
+    session.commit()
+
+    cat = RuleBasedCategorizer(session)
+    txn = Transaction(date=datetime.date(2026, 1, 15), amount=-48.52, description="WAL-MART #7181", account_id=1)
+    result = cat.categorize(txn)
+    assert result.category_id == groceries.id
+
+
+def test_tag_override():
+    session = make_session()
+    seed_categories(session)
+    dining = session.query(Category).filter_by(name="Dining Out").one()
+    rule = CategorizationRule(pattern="CHICK-FIL-A", category_id=dining.id, tag_override="needs", priority=10)
+    session.add(rule)
+    session.commit()
+
+    cat = RuleBasedCategorizer(session)
+    txn = Transaction(date=datetime.date(2026, 1, 15), amount=-5.39, description="CHICK-FIL-A #01476", account_id=1)
+    result = cat.categorize(txn)
+    assert result.tag == "needs"
+
+
+def test_person_attribution():
+    session = make_session()
+    seed_categories(session)
+    donna = HouseholdMember(name="Donna", relationship="wife")
+    session.add(donna)
+    session.flush()
+    income = session.query(Category).filter_by(name="Income").one()
+    rule = CategorizationRule(pattern="OASISBATCH PAYROLL", category_id=income.id, attributed_to_id=donna.id, priority=10)
+    session.add(rule)
+    session.commit()
+
+    cat = RuleBasedCategorizer(session)
+    txn = Transaction(date=datetime.date(2026, 1, 9), amount=1087.83, description="OASISBATCH PAYROLL 260109 DONNA CONLON", account_id=1)
+    result = cat.categorize(txn)
+    assert result.category_id == income.id
+    assert result.attributed_to_id == donna.id