diff --git a/backend/test/mastodon/oauth.spec.ts b/backend/test/mastodon/oauth.spec.ts
index 765e47f..669da1b 100644
--- a/backend/test/mastodon/oauth.spec.ts
+++ b/backend/test/mastodon/oauth.spec.ts
@@ -174,6 +174,31 @@ describe('Mastodon APIs', () => {
 			assert((await getSigningKey(userKEK, db, actor as Actor)) instanceof CryptoKey)
 		})
 
+		test('first login redirect relative URLs', async () => {
+			const db = await makeDB()
+
+			const params = new URLSearchParams({
+				redirect_uri: '/a',
+			})
+
+			const formData = new FormData()
+			formData.set('username', 'username')
+			formData.set('name', 'name')
+
+			const req = new Request('https://example.com/first-login?' + params, {
+				method: 'POST',
+				body: formData,
+				headers: {
+					cookie: `CF_Authorization=${TEST_JWT}`,
+				},
+			})
+			const res = await first_login.handlePostRequest(req, db, userKEK, accessDomain, accessAud)
+			assert.equal(res.status, 302)
+
+			const location = res.headers.get('location')
+			assert.equal(location, 'https://example.com/a')
+		})
+
 		test('token error on unknown client', async () => {
 			const db = await makeDB()
 			const body = new URLSearchParams({ code: 'some-code' })
diff --git a/functions/first-login.ts b/functions/first-login.ts
index 1a9c9e2..a4e95ea 100644
--- a/functions/first-login.ts
+++ b/functions/first-login.ts
@@ -55,6 +55,10 @@ export async function handlePostRequest(
 		return new Response('', { status: 400 })
 	}
 
-	const redirect_uri = decodeURIComponent(url.searchParams.get('redirect_uri') || '')
+	let redirect_uri = decodeURIComponent(url.searchParams.get('redirect_uri') || '')
+	if (redirect_uri.startsWith('/')) {
+		// URL is a relative URL, prepend the domain to it.
+		redirect_uri = 'https://' + url.hostname + redirect_uri
+	}
 	return Response.redirect(redirect_uri, 302)
 }