diff --git a/feature-list.md b/feature-list.md
index e8c605e..b6c9aef 100644
--- a/feature-list.md
+++ b/feature-list.md
@@ -6,6 +6,7 @@ Features to implement over time, roughly in priority order.
 
 - **Calendar event responses** — parse `text/calendar` parts from incoming emails and surface Accept / Decline / Tentative actions inline in the email view
 - **Paste images into composer** — handle `paste` events in the composer, upload the image blob to Fastmail via JMAP `Upload`, insert a `![](cid:...)` reference or inline data URL into the markdown, and embed it correctly in the outgoing multipart email
+- **Mark as unread** — button in the email detail view that removes `$seen` via `Email/set` and returns to inbox
 
 ## Ideas (not yet prioritized)
 
@@ -17,7 +18,7 @@ Features to implement over time, roughly in priority order.
 - Draft auto-save
 - Multiple account support (Gmail, Outlook via IMAP bridge or OAuth)
 - Unread count badge in sidebar
-- Mark as read / unread / starred
+- Mark as starred
 - Archive and delete actions
 - Mobile-responsive layout
 - Dark mode
diff --git a/src/app/email/[id]/page.tsx b/src/app/email/[id]/page.tsx
index ad73cbd..3994fbd 100644
--- a/src/app/email/[id]/page.tsx
+++ b/src/app/email/[id]/page.tsx
@@ -1,5 +1,5 @@
 import Link from "next/link";
-import { getSession, getAccountId, getEmail } from "@/lib/jmap";
+import { getSession, getAccountId, getEmail, markAsRead } from "@/lib/jmap";
 import { formatAddressList, formatFullDate } from "@/lib/format";
 import { notFound } from "next/navigation";
 import EmailBody from "@/components/EmailBody";
@@ -18,6 +18,11 @@ export default async function EmailPage({ params }: Props) {
 
   if (!email) return notFound();
 
+  // Mark as read if not already seen — fire-and-forget, don't block render
+  if (!email.keywords?.["$seen"]) {
+    void markAsRead(session.apiUrl, accountId, id);
+  }
+
   let body: string | null = null;
   let bodyType: "html" | "text" = "text";
 
diff --git a/src/lib/jmap.ts b/src/lib/jmap.ts
index 31aefdf..7724f33 100644
--- a/src/lib/jmap.ts
+++ b/src/lib/jmap.ts
@@ -153,6 +153,23 @@ export async function getIdentities(
   return (result.list as Identity[]) ?? [];
 }
 
+export async function markAsRead(
+  apiUrl: string,
+  accountId: string,
+  emailId: string
+): Promise<void> {
+  await jmapCall(apiUrl, [
+    [
+      "Email/set",
+      {
+        accountId,
+        update: { [emailId]: { "keywords/$seen": true } },
+      },
+      "0",
+    ],
+  ]);
+}
+
 export async function sendEmail(
   apiUrl: string,
   accountId: string,