diff --git a/internal/webserver/handler_messages.go b/internal/webserver/handler_messages.go
index 9103055..aff75e4 100644
--- a/internal/webserver/handler_messages.go
+++ b/internal/webserver/handler_messages.go
@@ -25,6 +25,30 @@ func (app *Application) messages_index(w http.ResponseWriter, r *http.Request) {
 	app.buffered_render_page(w, "tpl/messages.tpl", global_data, chat_view_data)
 }
 
+func (app *Application) message_mark_as_read(w http.ResponseWriter, r *http.Request) {
+	room_id := get_room_id_from_context(r.Context())
+
+	c := persistence.NewConversationCursor(room_id)
+	c.PageSize = 1
+	chat_contents := app.Profile.GetChatRoomMessagesByCursor(c)
+	last_message_id := chat_contents.MessageIDs[len(chat_contents.MessageIDs)-1]
+	scraper.MarkDMChatRead(room_id, last_message_id)
+	room := chat_contents.Rooms[room_id]
+	participant, is_ok := room.Participants[app.ActiveUser.ID]
+	if !is_ok {
+		panic(room)
+	}
+	participant.LastReadEventID = last_message_id
+	room.Participants[app.ActiveUser.ID] = participant
+	panic_if(app.Profile.SaveChatRoom(room))
+	app.toast(w, r, Toast{
+		Title:          "Success",
+		Message:        `Conversation marked as "read"`,
+		Type:           "success",
+		AutoCloseDelay: 2000,
+	})
+}
+
 func (app *Application) message_send(w http.ResponseWriter, r *http.Request) {
 	room_id := get_room_id_from_context(r.Context())
 
@@ -48,6 +72,11 @@ func (app *Application) message_detail(w http.ResponseWriter, r *http.Request) {
 
 	chat_view_data, global_data := app.get_message_global_data()
 
+	if len(parts) == 1 && parts[0] == "mark-as-read" {
+		app.message_mark_as_read(w, r)
+		return
+	}
+
 	// First send a message, if applicable
 	if is_sending {
 		app.message_send(w, r)
diff --git a/internal/webserver/static/styles.css b/internal/webserver/static/styles.css
index 4ae79d5..c135039 100644
--- a/internal/webserver/static/styles.css
+++ b/internal/webserver/static/styles.css
@@ -184,6 +184,11 @@ h3 {
 	height: auto;
 }
 
+.disappearing {
+    transition: opacity 2s ease;
+    opacity: 0;
+}
+
 
 /***************************************************************************************************
  *
diff --git a/internal/webserver/tpl/tweet_page_includes/chat_view.tpl b/internal/webserver/tpl/tweet_page_includes/chat_view.tpl
index 7798502..72b8f8e 100644
--- a/internal/webserver/tpl/tweet_page_includes/chat_view.tpl
+++ b/internal/webserver/tpl/tweet_page_includes/chat_view.tpl
@@ -131,6 +131,9 @@
           <a class="button" href="https://twitter.com/messages/{{ $room.ID }}" target="_blank">
             <img class="svg-icon" src="/static/icons/external-link.svg" width="24" height="24" />
           </a>
+          <a class="button" hx-post="/messages/{{ $room.ID }}/mark-as-read">
+            <img class="svg-icon" src="/static/icons/external-link.svg" width="24" height="24" />
+          </a>
         </div>
       </div>
     {{end}}
diff --git a/pkg/scraper/api_request_utils.go b/pkg/scraper/api_request_utils.go
index b277a22..eff25a7 100644
--- a/pkg/scraper/api_request_utils.go
+++ b/pkg/scraper/api_request_utils.go
@@ -164,7 +164,11 @@ func (api *API) do_http_POST(remote_url string, body string, result interface{})
 		return fmt.Errorf("Error initializing HTTP POST request:\n  %w", err)
 	}
 
-	req.Header.Set("content-type", "application/json")
+	if body[0] == '{' {
+		req.Header.Set("content-type", "application/json")
+	} else {
+		req.Header.Set("content-type", "application/x-www-form-urlencoded")
+	}
 
 	api.add_authentication_headers(req)
 
@@ -191,6 +195,11 @@ func (api *API) do_http_POST(remote_url string, body string, result interface{})
 		panic(err)
 	}
 
+	if resp.StatusCode == 204 {
+		// No Content
+		return nil
+	}
+
 	if resp.StatusCode != 200 {
 		responseHeaders := ""
 		for header := range resp.Header {
diff --git a/pkg/scraper/api_types_dms.go b/pkg/scraper/api_types_dms.go
index 9988048..faa2243 100644
--- a/pkg/scraper/api_types_dms.go
+++ b/pkg/scraper/api_types_dms.go
@@ -521,3 +521,16 @@ func (api *API) SendDMMessage(room_id DMChatRoomID, text string, in_reply_to_id
 	err = api.do_http_POST(url.String(), post_data, &result)
 	return result, err
 }
+
+// Mark a chat as read.
+func (api *API) MarkDMChatRead(room_id DMChatRoomID, read_message_id DMMessageID) {
+	url := fmt.Sprintf("https://twitter.com/i/api/1.1/dm/conversation/%s/mark_read.json", room_id)
+
+	// `do_http_POST` will set the "content-type" header based on whether the body starts with '{' or not.
+	data := fmt.Sprintf("conversationId=%s&last_read_event_id=%d", room_id, read_message_id)
+
+	err := api.do_http_POST(url, data, nil) // Expected: HTTP 204
+	if err != nil {
+		panic(err)
+	}
+}
diff --git a/pkg/scraper/dm_trove.go b/pkg/scraper/dm_trove.go
index 1f8d892..88bf1a0 100644
--- a/pkg/scraper/dm_trove.go
+++ b/pkg/scraper/dm_trove.go
@@ -111,3 +111,9 @@ func SendDMMessage(room_id DMChatRoomID, text string, in_reply_to_id DMMessageID
 	}
 	return dm_response.ToDMTrove()
 }
+func MarkDMChatRead(room_id DMChatRoomID, read_message_id DMMessageID) {
+	if !the_api.IsAuthenticated {
+		log.Fatalf("Writing DMs can only be done when authenticated.  Please provide `--session [user]`")
+	}
+	the_api.MarkDMChatRead(room_id, read_message_id)
+}