diff --git a/app/api/me/timezone/route.ts b/app/api/me/timezone/route.ts
new file mode 100644
index 0000000..98915bd
--- /dev/null
+++ b/app/api/me/timezone/route.ts
@@ -0,0 +1,103 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { requireAuth } from '@/lib/auth-utils';
+import { postgresClient } from '@/lib/services/postgres-client';
+
+// GET  /api/me/timezone   -> { timezone: string, source: 'user' | 'default' }
+// PUT  /api/me/timezone   -> body { timezone: string } -> { timezone: string }
+//
+// TZ-03. Authentication: requireAuth(). The PUT handler updates ONLY the
+// calling user's row — there is no `userId` query param or body field. The
+// write target is always `session.user.id`.
+//
+// Validation: the input timezone must appear in
+// `Intl.supportedValuesOf('timeZone')`. Anything else is rejected with 400
+// before touching the database.
+
+function getDefaultTimezone(): string {
+  return process.env.DEFAULT_TIMEZONE || 'UTC';
+}
+
+function isValidIanaTimezone(tz: unknown): tz is string {
+  if (typeof tz !== 'string' || tz.length === 0 || tz.length > 64) return false;
+  try {
+    const zones = Intl.supportedValuesOf('timeZone');
+    return zones.includes(tz);
+  } catch {
+    return false;
+  }
+}
+
+export async function GET(): Promise<NextResponse> {
+  const { session, error } = await requireAuth();
+  if (error) return error;
+
+  try {
+    const result = await postgresClient.query<{ timezone: string | null }>(
+      'SELECT timezone FROM "user" WHERE id = $1',
+      [session!.user.id],
+    );
+    const stored = result.rows[0]?.timezone;
+    const fallback = getDefaultTimezone();
+    const timezone = stored && stored.length > 0 ? stored : fallback;
+    const source: 'user' | 'default' =
+      stored && stored.length > 0 && stored !== fallback ? 'user' : 'default';
+    return NextResponse.json({ timezone, source });
+  } catch (e) {
+    console.error('GET /api/me/timezone failed:', e);
+    return NextResponse.json(
+      { error: 'Failed to read timezone', message: e instanceof Error ? e.message : 'unknown' },
+      { status: 500 },
+    );
+  }
+}
+
+export async function PUT(request: NextRequest): Promise<NextResponse> {
+  const { session, error } = await requireAuth();
+  if (error) return error;
+
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json(
+      { error: 'Invalid JSON', message: 'Request body must be JSON' },
+      { status: 400 },
+    );
+  }
+
+  const candidate =
+    body && typeof body === 'object' && 'timezone' in body
+      ? (body as { timezone: unknown }).timezone
+      : undefined;
+
+  if (!isValidIanaTimezone(candidate)) {
+    return NextResponse.json(
+      {
+        error: 'Invalid timezone',
+        message: "timezone must be an IANA zone present in Intl.supportedValuesOf('timeZone')",
+      },
+      { status: 400 },
+    );
+  }
+
+  try {
+    // Authoritative write target: session.user.id. NO userId from body.
+    const result = await postgresClient.query<{ timezone: string }>(
+      'UPDATE "user" SET timezone = $1, updated_at = NOW() WHERE id = $2 RETURNING timezone',
+      [candidate, session!.user.id],
+    );
+    if (result.rowCount === 0) {
+      return NextResponse.json(
+        { error: 'User not found', message: 'No user row matched the session' },
+        { status: 404 },
+      );
+    }
+    return NextResponse.json({ timezone: result.rows[0].timezone });
+  } catch (e) {
+    console.error('PUT /api/me/timezone failed:', e);
+    return NextResponse.json(
+      { error: 'Failed to update timezone', message: e instanceof Error ? e.message : 'unknown' },
+      { status: 500 },
+    );
+  }
+}