diff --git a/CHANGES.md b/CHANGES.md
index 7539f2e..010b6d9 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Billing: CSV export of a tenant's invoices (`GET /billing/invoices.csv`), with optional `from`, `to` and `currency` filters.
 
 ## 3.2.0
 - Returns: partial refunds.
diff --git a/src/modules/billing/formatters.js b/src/modules/billing/formatters.js
index f64ce28..8cac6c6 100644
--- a/src/modules/billing/formatters.js
+++ b/src/modules/billing/formatters.js
@@ -1,5 +1,27 @@
 import { formatCents } from '../../lib/money.js';
+import { isoDate } from '../../lib/time.js';
 
 export function formatInvoice(inv) {
   return { ...inv, total: formatCents(inv.total) };
 }
+
+const CSV_COLUMNS = ['id', 'date', 'customer', 'subtotal', 'discount', 'tax', 'total', 'currency'];
+
+function csvField(v) {
+  const s = String(v);
+  return /[",\r\n]/.test(s) ? `"${s.replace(/"/g, '""')}"` : s;
+}
+
+export function formatInvoicesCsv(invoices) {
+  const lines = invoices.map((i) => [
+    i.id,
+    isoDate(i.date),
+    i.customer,
+    formatCents(i.subtotal),
+    formatCents(i.discount),
+    formatCents(i.tax),
+    formatCents(i.total),
+    i.currency,
+  ].map(csvField).join(','));
+  return [CSV_COLUMNS.join(','), ...lines].join('\n') + '\n';
+}
diff --git a/src/modules/billing/routes.js b/src/modules/billing/routes.js
index 0ac3d5c..e57b9ef 100644
--- a/src/modules/billing/routes.js
+++ b/src/modules/billing/routes.js
@@ -1,8 +1,9 @@
-import { ok, wrap } from '../../lib/http.js';
-import { getInvoice, listInvoices } from './service.js';
-import { formatInvoice } from './formatters.js';
+import { ok, text, wrap } from '../../lib/http.js';
+import { exportInvoices, getInvoice, listInvoices } from './service.js';
+import { formatInvoice, formatInvoicesCsv } from './formatters.js';
 
 export const routes = [
   ['GET', '/billing/invoices', wrap(async (req) => ok(listInvoices(req.query).map(formatInvoice)))],
+  ['GET', '/billing/invoices.csv', wrap(async (req) => text(formatInvoicesCsv(exportInvoices(req.query)), 'text/csv'))],
   ['GET', '/billing/invoices/:id', wrap(async (req) => ok(formatInvoice(getInvoice(req.params.id))))],
 ];
diff --git a/src/modules/billing/service.js b/src/modules/billing/service.js
index d84492f..2a5bef1 100644
--- a/src/modules/billing/service.js
+++ b/src/modules/billing/service.js
@@ -1,4 +1,7 @@
-import { NotFoundError } from '../../lib/errors.js';
+import { NotFoundError, ValidationError } from '../../lib/errors.js';
+import { requireString } from '../../lib/validate.js';
+import { inRange, isoDate } from '../../lib/time.js';
+import { validateCurrency } from './validators.js';
 import { discountFor } from './discounts.js';
 import { taxFor } from './tax.js';
 import { TAX_REGION } from './constants.js';
@@ -21,3 +24,25 @@ export function getInvoice(id) {
 export function listInvoices({ tenantId } = {}) {
   return allInvoices().filter((i) => !tenantId || i.tenantId === tenantId);
 }
+
+function optionalDate(query, key) {
+  const v = query?.[key];
+  if (v === undefined || v === '') return undefined;
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(v) || isNaN(new Date(v)) || isoDate(v) !== v) {
+    throw new ValidationError(`${key} must be a date in YYYY-MM-DD format`);
+  }
+  return v;
+}
+
+// Invoices for the CSV export: raw integer cents, filtered by inclusive date range, sorted by date then id.
+export function exportInvoices(query = {}) {
+  const tenant = requireString(query, 'tenant');
+  const from = optionalDate(query, 'from');
+  const to = optionalDate(query, 'to');
+  const currency = query.currency === undefined ? undefined : validateCurrency(query);
+  const all = listInvoices({ tenantId: tenant });
+  if (all.length === 0) throw new NotFoundError('invoices for tenant ' + tenant);
+  return all
+    .filter((i) => inRange(i.date, from, to) && (!currency || i.currency === currency))
+    .sort((a, b) => (a.date < b.date ? -1 : a.date > b.date ? 1 : a.id < b.id ? -1 : a.id > b.id ? 1 : 0));
+}
diff --git a/tests/billing.test.js b/tests/billing.test.js
index 445b393..e0f8991 100644
--- a/tests/billing.test.js
+++ b/tests/billing.test.js
@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { priceLines, getInvoice, listInvoices, discountFor } from '../src/modules/billing/index.js';
+import { priceLines, getInvoice, listInvoices, discountFor, exportInvoices, formatInvoicesCsv } from '../src/modules/billing/index.js';
 
 test('billing: prices lines with discount and tax', () => {
   const p = priceLines([{ unitCents: 1000, qty: 2 }], { discountCode: 'save10', region: 'US' });
@@ -17,3 +17,31 @@ test('billing: invoices', () => {
   assert.equal(listInvoices({ tenantId: 'globex' }).length, 1);
   assert.throws(() => getInvoice('inv_9999'), /not found/);
 });
+
+test('billing: exportInvoices filters, sorts and validates', () => {
+  assert.deepEqual(exportInvoices({ tenant: 'acme' }).map((i) => i.id),
+    ['inv_0006', 'inv_0001', 'inv_0002', 'inv_0003', 'inv_0004']);
+  assert.deepEqual(exportInvoices({ tenant: 'acme', from: '2026-09-15', to: '2026-09-15' }).map((i) => i.id),
+    ['inv_0002', 'inv_0003']);
+  assert.deepEqual(exportInvoices({ tenant: 'acme', from: '2027-01-01' }), []);
+  assert.deepEqual(exportInvoices({ tenant: 'acme', currency: 'USD' }).map((i) => i.id),
+    ['inv_0006', 'inv_0001', 'inv_0002', 'inv_0004']);
+  assert.deepEqual(exportInvoices({ tenant: 'acme', currency: 'EUR' }).map((i) => i.id), ['inv_0003']);
+  assert.deepEqual(exportInvoices({ tenant: 'acme', currency: 'GBP' }), []);
+  assert.throws(() => exportInvoices({ tenant: 'acme', currency: 'JPY' }), /currency must be one of/);
+  assert.throws(() => exportInvoices({ tenant: 'acme', currency: 'usd' }), /currency must be one of/);
+  assert.throws(() => exportInvoices({ tenant: 'acme', currency: '' }), /currency must be one of/);
+  assert.throws(() => exportInvoices({}), /tenant is required/);
+  assert.throws(() => exportInvoices({ tenant: 'acme', from: '2026-02-30' }), /from must be/);
+  assert.throws(() => exportInvoices({ tenant: 'acme', to: 'garbage' }), /to must be/);
+  assert.throws(() => exportInvoices({ tenant: 'nobody' }), /not found/);
+});
+
+test('billing: formatInvoicesCsv quotes fields and ends with newline', () => {
+  const header = 'id,date,customer,subtotal,discount,tax,total,currency\n';
+  assert.equal(formatInvoicesCsv([]), header);
+  const csv = formatInvoicesCsv([
+    { id: 'a', date: '2026-01-02', customer: 'x,y "z"\nw', currency: 'USD', subtotal: 105, discount: 0, tax: 7, total: 112 },
+  ]);
+  assert.equal(csv, header + 'a,2026-01-02,"x,y ""z""\nw",1.05,0.00,0.07,1.12,USD\n');
+});
diff --git a/tests/server.test.js b/tests/server.test.js
index e836eaa..b336dec 100644
--- a/tests/server.test.js
+++ b/tests/server.test.js
@@ -17,3 +17,27 @@ test('server: invoice route and 404s', async () => {
   assert.equal((await app.handle('GET', '/billing/invoices/nope')).status, 404);
   assert.equal((await app.handle('GET', '/nothing')).status, 404);
 });
+
+test('server: invoices csv export', async () => {
+  const app = createApp();
+  const header = 'id,date,customer,subtotal,discount,tax,total,currency\n';
+  const r = await app.handle('GET', '/billing/invoices.csv?tenant=acme');
+  assert.equal(r.status, 200);
+  assert.equal(r.headers['content-type'], 'text/csv');
+  assert.equal(r.body, header +
+    'inv_0006,2026-08-31,Blue Fern Ltd,30.00,0.00,2.10,32.10,USD\n' +
+    'inv_0001,2026-09-02,Blue Fern Ltd,120.00,12.00,7.56,115.56,USD\n' +
+    'inv_0002,2026-09-15,"Smith, Jones & Co",49.99,0.00,3.50,53.49,USD\n' +
+    'inv_0003,2026-09-15,"The ""Quick"" Shop",200.00,5.00,39.00,234.00,EUR\n' +
+    'inv_0004,2026-10-01,Ortega Freight,1.05,0.00,0.07,1.12,USD\n');
+  const ranged = await app.handle('GET', '/billing/invoices.csv?tenant=acme&from=2026-09-15&to=2026-09-15');
+  assert.deepEqual(ranged.body.split('\n').slice(1, -1).map((l) => l.slice(0, 8)), ['inv_0002', 'inv_0003']);
+  assert.equal((await app.handle('GET', '/billing/invoices.csv?tenant=acme&from=2027-01-01')).body, header);
+  const eur = await app.handle('GET', '/billing/invoices.csv?tenant=acme&currency=EUR');
+  assert.equal(eur.body, header + 'inv_0003,2026-09-15,"The ""Quick"" Shop",200.00,5.00,39.00,234.00,EUR\n');
+  assert.equal((await app.handle('GET', '/billing/invoices.csv?tenant=acme&currency=GBP')).body, header);
+  assert.equal((await app.handle('GET', '/billing/invoices.csv?tenant=acme&currency=JPY')).status, 400);
+  assert.equal((await app.handle('GET', '/billing/invoices.csv')).status, 400);
+  assert.equal((await app.handle('GET', '/billing/invoices.csv?tenant=acme&from=2026-02-30')).status, 400);
+  assert.equal((await app.handle('GET', '/billing/invoices.csv?tenant=nobody')).status, 404);
+});