diff --git a/backend/app/domains/storefronts/__init__.py b/backend/app/domains/storefronts/__init__.py
new file mode 100644
index 0000000..d2db7f1
--- /dev/null
+++ b/backend/app/domains/storefronts/__init__.py
@@ -0,0 +1,19 @@
+"""storefronts domain — storefront entity, membership, the one-storefront guard.
+
+Owns INV-4 (one storefront per account is a service-layer rule) and INV-5 (tenant rows
+carry storefront_id). Knows nothing about identity (that is the accounts domain). Imported
+via this package surface only (§6.2).
+"""
+from __future__ import annotations
+
+from .errors import AlreadyOwnsStorefront, StorefrontsError
+from .models import Storefront
+from .service import create_storefront, storefront_for
+
+__all__ = [
+    "Storefront",
+    "StorefrontsError",
+    "AlreadyOwnsStorefront",
+    "create_storefront",
+    "storefront_for",
+]
diff --git a/backend/app/domains/storefronts/errors.py b/backend/app/domains/storefronts/errors.py
new file mode 100644
index 0000000..699f1a7
--- /dev/null
+++ b/backend/app/domains/storefronts/errors.py
@@ -0,0 +1,10 @@
+"""storefronts domain errors."""
+from __future__ import annotations
+
+
+class StorefrontsError(Exception):
+    """Base for storefronts-domain errors."""
+
+
+class AlreadyOwnsStorefront(StorefrontsError):
+    """INV-4: the account already has its one storefront (PUC-7)."""
diff --git a/backend/app/domains/storefronts/models.py b/backend/app/domains/storefronts/models.py
new file mode 100644
index 0000000..4bedb2f
--- /dev/null
+++ b/backend/app/domains/storefronts/models.py
@@ -0,0 +1,10 @@
+"""Storefront record — the §6.3 entity as the domain returns it."""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Storefront:
+    id: int
+    name: str
diff --git a/backend/app/domains/storefronts/service.py b/backend/app/domains/storefronts/service.py
new file mode 100644
index 0000000..4249dc2
--- /dev/null
+++ b/backend/app/domains/storefronts/service.py
@@ -0,0 +1,56 @@
+"""storefronts — the storefront + membership service (SD-0001 §6.5).
+
+Owns the storefront entity, the account<->storefront membership (INV-5), the entry-routing
+answer (storefront_for), and INV-4's one-storefront guard — the single deletable check that
+makes one-per-account an MVP rule, not a schema law. Never mints identity: the BFF passes
+account_id and email down.
+"""
+from __future__ import annotations
+
+import psycopg
+
+from .errors import AlreadyOwnsStorefront
+from .models import Storefront
+
+
+def _default_name(email: str) -> str:
+    """§6.3: a blank name stores a generated default — never NULL (corpus 14.01.0026)."""
+    return f"{email.split('@', 1)[0]}'s storefront"
+
+
+def storefront_for(conn: psycopg.Connection, account_id: int) -> Storefront | None:
+    """The entry-routing answer (§6.5): which storefront, if any, this account has."""
+    row = conn.execute(
+        "SELECT s.id, s.name FROM storefront s"
+        " JOIN storefront_membership m ON m.storefront_id = s.id"
+        " WHERE m.account_id = %s ORDER BY m.created_at LIMIT 1",
+        (account_id,),
+    ).fetchone()
+    return Storefront(id=row[0], name=row[1]) if row else None
+
+
+def create_storefront(
+    conn: psycopg.Connection, account_id: int, email: str, name: str | None
+) -> Storefront:
+    """Create the account's one storefront + owner membership (PUC-4; INV-4, INV-5).
+
+    Guard + insert run as one atomic unit: a transaction-scoped advisory lock keyed by
+    account_id serializes concurrent creates for the same account, so the second of two
+    racing requests sees the first's committed membership and is refused (§6.5).
+    """
+    conn.execute("SELECT pg_advisory_xact_lock(%s)", (account_id,))
+    existing = storefront_for(conn, account_id)
+    if existing is not None:
+        conn.rollback()  # release the advisory lock; nothing was written
+        raise AlreadyOwnsStorefront()
+    final_name = (name or "").strip() or _default_name(email)
+    sf_id = conn.execute(
+        "INSERT INTO storefront (name) VALUES (%s) RETURNING id", (final_name,)
+    ).fetchone()[0]
+    conn.execute(
+        "INSERT INTO storefront_membership (account_id, storefront_id, role)"
+        " VALUES (%s, %s, 'owner')",
+        (account_id, sf_id),
+    )
+    conn.commit()
+    return Storefront(id=sf_id, name=final_name)
diff --git a/backend/tests/test_storefronts_create.py b/backend/tests/test_storefronts_create.py
new file mode 100644
index 0000000..e0a2be2
--- /dev/null
+++ b/backend/tests/test_storefronts_create.py
@@ -0,0 +1,64 @@
+"""SLICE-3 storefront creation — service + endpoint scenario tests (SD-0001 §6.5 PUC-4/7)."""
+import re
+
+import psycopg
+import pytest
+from fastapi.testclient import TestClient
+
+from app.main import create_app
+from app.domains import storefronts
+from app.platform import db
+
+
+@pytest.fixture()
+def migrated_conn(fresh_db_url):
+    with psycopg.connect(fresh_db_url) as conn:
+        db.migrate(conn)
+        yield conn
+
+
+def _account_id(conn, email="merchant@example.com"):
+    return conn.execute(
+        "INSERT INTO account (email) VALUES (%s) RETURNING id", (email,)
+    ).fetchone()[0]
+
+
+def test_create_storefront_with_name(migrated_conn):
+    acct = _account_id(migrated_conn)
+    sf = storefronts.create_storefront(migrated_conn, acct, "merchant@example.com", "Ben's Bets")
+    assert sf.name == "Ben's Bets"
+    assert storefronts.storefront_for(migrated_conn, acct) == sf
+
+
+def test_14_01_0026_blank_name_gets_generated_default(migrated_conn):
+    acct = _account_id(migrated_conn)
+    sf = storefronts.create_storefront(migrated_conn, acct, "merchant@example.com", None)
+    assert sf.name == "merchant's storefront"
+
+
+def test_whitespace_name_treated_as_blank(migrated_conn):
+    acct = _account_id(migrated_conn)
+    sf = storefronts.create_storefront(migrated_conn, acct, "ben@bensbets.com", "   ")
+    assert sf.name == "ben's storefront"
+
+
+def test_second_create_refused_inv_4(migrated_conn):
+    acct = _account_id(migrated_conn)
+    storefronts.create_storefront(migrated_conn, acct, "merchant@example.com", "First")
+    with pytest.raises(storefronts.AlreadyOwnsStorefront):
+        storefronts.create_storefront(migrated_conn, acct, "merchant@example.com", "Second")
+
+
+def test_membership_row_is_owner(migrated_conn):
+    acct = _account_id(migrated_conn)
+    sf = storefronts.create_storefront(migrated_conn, acct, "merchant@example.com", None)
+    role = migrated_conn.execute(
+        "SELECT role FROM storefront_membership WHERE account_id = %s AND storefront_id = %s",
+        (acct, sf.id),
+    ).fetchone()[0]
+    assert role == "owner"
+
+
+def test_storefront_for_none_when_absent(migrated_conn):
+    acct = _account_id(migrated_conn)
+    assert storefronts.storefront_for(migrated_conn, acct) is None