diff --git a/apps/api/src/auth.ts b/apps/api/src/auth.ts
index d85bc3c2..92e67649 100644
--- a/apps/api/src/auth.ts
+++ b/apps/api/src/auth.ts
@@ -8,6 +8,7 @@ import {
 import { autumn } from "autumn-js/better-auth";
 import { betterAuth } from "better-auth";
 import { drizzleAdapter } from "better-auth/adapters/drizzle";
+import { APIError } from "better-auth/api";
 import { admin, organization } from "better-auth/plugins";
 import { eq } from "drizzle-orm";
 import { favoriteProjectToSeed, pickActiveOrgId as pickActiveOrgIdRule } from "./active-context.js";
@@ -20,6 +21,7 @@ import {
   verificationEmailBody,
 } from "./email.js";
 import { readSignupAttributionFromCookieHeader } from "./signup-click-ids.js";
+import { blockedSignupEmailDomains, isSignupEmailBlocked } from "./signup-email-domains.js";
 import { enqueueUserCreated } from "./user-created-publisher.js";
 import { configuredWebOrigins } from "./web-origins.js";
 
@@ -68,6 +70,8 @@ function sharedCookieDomain(): string | undefined {
 
 const COOKIE_DOMAIN = sharedCookieDomain();
 
+const BLOCKED_SIGNUP_EMAIL_DOMAINS = blockedSignupEmailDomains();
+
 // Autumn billing plugin. Maps each org (a Better Auth "organization") to an
 // Autumn customer, auto-created on first sign-in, so billing is per-org. Only
 // enabled when AUTUMN_SECRET_KEY is present, so local dev / worktrees without
@@ -273,6 +277,17 @@ export const auth = betterAuth({
     },
     user: {
       create: {
+        // Runs before every new users row — email, social, and admin-created
+        // alike — so a blocked domain can't slip in through an OAuth provider.
+        before: async (user) => {
+          if (isSignupEmailBlocked(user.email, BLOCKED_SIGNUP_EMAIL_DOMAINS)) {
+            console.warn("[auth] rejected sign-up from blocked email domain");
+            throw new APIError("FORBIDDEN", {
+              message: "Sign-ups from this email domain are not allowed.",
+            });
+          }
+          return { data: user };
+        },
         after: async (user, context) => {
           // Fires on every new users row — email and social signups alike, and
           // users who never finish onboarding — so the signup count is captured
diff --git a/apps/api/src/signup-email-domains.test.ts b/apps/api/src/signup-email-domains.test.ts
new file mode 100644
index 00000000..4f1cd114
--- /dev/null
+++ b/apps/api/src/signup-email-domains.test.ts
@@ -0,0 +1,39 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { blockedSignupEmailDomains, isSignupEmailBlocked } from "./signup-email-domains.js";
+
+test("no domains are blocked unless the deployment configures some", () => {
+  assert.deepEqual(blockedSignupEmailDomains({}), []);
+  assert.deepEqual(blockedSignupEmailDomains({ SIGNUP_BLOCKED_EMAIL_DOMAINS: " , " }), []);
+  assert.equal(isSignupEmailBlocked("someone@example.com", []), false);
+});
+
+test("parses a comma-separated, case-insensitive domain list", () => {
+  assert.deepEqual(
+    blockedSignupEmailDomains({ SIGNUP_BLOCKED_EMAIL_DOMAINS: " Spam.example ,@junk.test,," }),
+    ["spam.example", "junk.test"],
+  );
+});
+
+test("blocks addresses on a listed domain regardless of case", () => {
+  const blocked = ["spam.example"];
+  assert.equal(isSignupEmailBlocked("bot@spam.example", blocked), true);
+  assert.equal(isSignupEmailBlocked("Bot@SPAM.Example", blocked), true);
+  assert.equal(isSignupEmailBlocked("  bot@spam.example  ", blocked), true);
+});
+
+test("blocks subdomains of a listed domain", () => {
+  assert.equal(isSignupEmailBlocked("bot@mail.spam.example", ["spam.example"]), true);
+});
+
+test("does not block lookalike or unrelated domains", () => {
+  const blocked = ["spam.example"];
+  assert.equal(isSignupEmailBlocked("person@notspam.example", blocked), false);
+  assert.equal(isSignupEmailBlocked("person@spam.example.org", blocked), false);
+  assert.equal(isSignupEmailBlocked("spam.example@legit.test", blocked), false);
+});
+
+test("uses the domain after the last @", () => {
+  assert.equal(isSignupEmailBlocked('"a@legit.test"@spam.example', ["spam.example"]), true);
+  assert.equal(isSignupEmailBlocked("no-at-sign", ["spam.example"]), false);
+});
diff --git a/apps/api/src/signup-email-domains.ts b/apps/api/src/signup-email-domains.ts
new file mode 100644
index 00000000..8ec84c4a
--- /dev/null
+++ b/apps/api/src/signup-email-domains.ts
@@ -0,0 +1,22 @@
+type SignupEmailDomainEnv = {
+  SIGNUP_BLOCKED_EMAIL_DOMAINS?: string;
+};
+
+// Comma-separated list of email domains that may not create accounts (e.g.
+// a domain sending spam sign-ups). Subdomains of a listed domain are blocked
+// too. Unset = nothing blocked.
+export function blockedSignupEmailDomains(env: SignupEmailDomainEnv = process.env): string[] {
+  return (env.SIGNUP_BLOCKED_EMAIL_DOMAINS ?? "")
+    .split(",")
+    .map((value) => value.trim().replace(/^@/, "").toLowerCase())
+    .filter((value) => value.length > 0);
+}
+
+export function isSignupEmailBlocked(email: string, blockedDomains: readonly string[]): boolean {
+  if (blockedDomains.length === 0) return false;
+  const normalized = email.trim().toLowerCase();
+  const at = normalized.lastIndexOf("@");
+  if (at === -1) return false;
+  const domain = normalized.slice(at + 1);
+  return blockedDomains.some((blocked) => domain === blocked || domain.endsWith(`.${blocked}`));
+}