diff --git a/app/core/seeds.py b/app/core/seeds.py
new file mode 100644
index 0000000..ba119d3
--- /dev/null
+++ b/app/core/seeds.py
@@ -0,0 +1,86 @@
+"""Default data seeding — EUR currency, 19% and 7% tax rates."""
+
+from __future__ import annotations
+
+import logging
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.currency import Currency
+from app.models.tax import TaxRate
+
+logger = logging.getLogger(__name__)
+
+
+async def seed_default_data(db: AsyncSession, tenant_id: uuid.UUID | None = None) -> None:
+    """Seed default currency and tax rates for a tenant.
+
+    Idempotent: checks if data already exists before inserting.
+    If tenant_id is None, seeds for all existing tenants.
+    """
+    from app.models.tenant import Tenant
+
+    if tenant_id is not None:
+        tenant_ids = [tenant_id]
+    else:
+        result = await db.execute(select(Tenant.id))
+        tenant_ids = [row[0] for row in result.all()]
+
+    for tid in tenant_ids:
+        await _seed_currencies(db, tid)
+        await _seed_tax_rates(db, tid)
+
+    await db.flush()
+    logger.info(f"Seeded default data for {len(tenant_ids)} tenant(s)")
+
+
+async def _seed_currencies(db: AsyncSession, tenant_id: uuid.UUID) -> None:
+    """Seed EUR as default currency if no currencies exist for this tenant."""
+    existing = await db.execute(
+        select(Currency).where(
+            Currency.tenant_id == tenant_id,
+            Currency.deleted_at.is_(None),
+        )
+    )
+    if existing.scalars().first() is not None:
+        return
+
+    eur = Currency(
+        tenant_id=tenant_id,
+        code="EUR",
+        name="Euro",
+        symbol="€",
+        is_default=True,
+    )
+    db.add(eur)
+    logger.info(f"Seeded EUR currency for tenant {tenant_id}")
+
+
+async def _seed_tax_rates(db: AsyncSession, tenant_id: uuid.UUID) -> None:
+    """Seed 19% and 7% tax rates if no tax rates exist for this tenant."""
+    existing = await db.execute(
+        select(TaxRate).where(
+            TaxRate.tenant_id == tenant_id,
+            TaxRate.deleted_at.is_(None),
+        )
+    )
+    if existing.scalars().first() is not None:
+        return
+
+    vat_19 = TaxRate(
+        tenant_id=tenant_id,
+        name="Mehrwertsteuer 19%",
+        rate=19.00,
+        is_default=True,
+    )
+    vat_7 = TaxRate(
+        tenant_id=tenant_id,
+        name="Mehrwertsteuer 7%",
+        rate=7.00,
+        is_default=False,
+    )
+    db.add(vat_19)
+    db.add(vat_7)
+    logger.info(f"Seeded 19% and 7% tax rates for tenant {tenant_id}")