diff --git a/web/src/stream_popover.js b/web/src/stream_popover.js
index 993357ffd8d64e..6650b421eb8007 100644
--- a/web/src/stream_popover.js
+++ b/web/src/stream_popover.js
@@ -3,12 +3,14 @@ import $ from "jquery";
 import assert from "minimalistic-assert";
 
 import render_inline_decorated_stream_name from "../templates/inline_decorated_stream_name.hbs";
+import render_message_moving_warning_banner from "../templates/modal_banner/message_moving_warning_banner.hbs";
 import render_move_topic_to_stream from "../templates/move_topic_to_stream.hbs";
 import render_left_sidebar_stream_actions_popover from "../templates/popovers/left_sidebar/left_sidebar_stream_actions_popover.hbs";
 
 import * as blueslip from "./blueslip";
 import * as browser_history from "./browser_history";
 import * as compose_actions from "./compose_actions";
+import * as compose_banner from "./compose_banner";
 import * as composebox_typeahead from "./composebox_typeahead";
 import * as dialog_widget from "./dialog_widget";
 import * as dropdown_widget from "./dropdown_widget";
@@ -27,6 +29,7 @@ import * as stream_data from "./stream_data";
 import * as stream_settings_api from "./stream_settings_api";
 import * as stream_settings_components from "./stream_settings_components";
 import * as stream_settings_ui from "./stream_settings_ui";
+import * as stream_topic_history from "./stream_topic_history";
 import * as sub_store from "./sub_store";
 import * as ui_report from "./ui_report";
 import * as ui_util from "./ui_util";
@@ -558,6 +561,7 @@ export async function build_move_topic_to_stream_popover(
         stream_widget_value = $(event.currentTarget).attr("data-unique-id");
 
         update_submit_button_disabled_state(stream_widget_value);
+        update_warning_banner();
         set_stream_topic_typeahead();
         render_selected_stream();
 
@@ -566,9 +570,55 @@ export async function build_move_topic_to_stream_popover(
         event.stopPropagation();
     }
 
+    function update_warning_banner() {
+        // The warning banner only shows when
+        // 1) submit button is NOT disabled, and
+        // 2) target topic is an existing topic, and
+        // 3) propagate_mode is NOT change_one
+
+        // submit button
+        if ($("#move_topic_modal .dialog_submit_button")[0].disabled) {
+            $("#move_topic_modal .move_topic_form_warning_container").html("");
+            return;
+        }
+
+        // target topic
+        if (stream_widget_value !== undefined) {
+            const {new_topic_name} = get_params_from_form();
+            if (
+                !stream_topic_history.stream_has_topic(
+                    Number.parseInt(stream_widget_value, 10),
+                    new_topic_name,
+                )
+            ) {
+                $("#move_topic_modal .move_topic_form_warning_container").html("");
+                return;
+            }
+        }
+
+        // propagate_mode
+        if ($("#move_topic_modal select.message_edit_topic_propagate").val() === "change_one") {
+            $("#move_topic_modal .move_topic_form_warning_container").html("");
+            return;
+        }
+
+        $("#move_topic_modal .move_topic_form_warning_container").html(
+            render_message_moving_warning_banner({
+                banner_type: compose_banner.WARNING,
+                hide_close_button: true,
+                classname: "message_moving_warning_banner",
+            }),
+        );
+    }
+
     function move_topic_post_render() {
         $("#move_topic_modal .dialog_submit_button").prop("disabled", true);
 
+        $("#move_topic_modal select.message_edit_topic_propagate")[0]?.addEventListener(
+            "change",
+            () => update_warning_banner(),
+        );
+
         const $topic_input = $("#move_topic_form .move_messages_edit_topic");
         move_topic_to_stream_topic_typeahead = composebox_typeahead.initialize_topic_edit_typeahead(
             $topic_input,
@@ -582,6 +632,7 @@ export async function build_move_topic_to_stream_popover(
             const select_stream_id = current_stream_id;
             $topic_input.on("input", () => {
                 update_submit_button_disabled_state(select_stream_id);
+                update_warning_banner();
             });
             return;
         }
@@ -610,6 +661,7 @@ export async function build_move_topic_to_stream_popover(
         $("#move_topic_to_stream_widget").prop("disabled", disable_stream_input);
         $("#move_topic_modal .move_messages_edit_topic").on("input", () => {
             update_submit_button_disabled_state(stream_widget_value);
+            update_warning_banner();
         });
     }
 
diff --git a/web/src/stream_topic_history.ts b/web/src/stream_topic_history.ts
index dc3157b81bb256..80b1cbf9913dda 100644
--- a/web/src/stream_topic_history.ts
+++ b/web/src/stream_topic_history.ts
@@ -86,6 +86,17 @@ export function stream_has_topics(stream_id: number): boolean {
     return history.has_topics();
 }
 
+export function stream_has_topic(stream_id: number, topic_name: string): boolean {
+    if (!stream_dict.has(stream_id)) {
+        return false;
+    }
+
+    const history = stream_dict.get(stream_id);
+    assert(history !== undefined);
+
+    return history.has_topic(topic_name);
+}
+
 export type TopicHistoryEntry = {
     count: number;
     message_id: number;
@@ -126,6 +137,10 @@ export class PerStreamHistory {
         return this.topics.size !== 0;
     }
 
+    has_topic(topic_name: string): boolean {
+        return this.topics.has(topic_name);
+    }
+
     update_stream_with_message_id(message_id: number): void {
         if (message_id > this.max_message_id) {
             this.max_message_id = message_id;
diff --git a/web/templates/modal_banner/message_moving_warning_banner.hbs b/web/templates/modal_banner/message_moving_warning_banner.hbs
new file mode 100644
index 00000000000000..ee88ec414e715c
--- /dev/null
+++ b/web/templates/modal_banner/message_moving_warning_banner.hbs
@@ -0,0 +1,7 @@
+{{#> modal_banner }}
+    <p class="banner_message">
+        {{#tr}}
+            You are moving messages to a topic that already exists. Messages from these topics will be combined.
+        {{/tr}}
+    </p>
+{{/modal_banner}}
diff --git a/web/templates/move_topic_to_stream.hbs b/web/templates/move_topic_to_stream.hbs
index 2441af76f922cf..eb9635df1974d4 100644
--- a/web/templates/move_topic_to_stream.hbs
+++ b/web/templates/move_topic_to_stream.hbs
@@ -1,7 +1,8 @@
 {{#unless (or from_message_actions_popover only_topic_edit)}}
 <p class="white-space-preserve-wrap">{{#tr}}Move all messages in <strong>{topic_name}</strong>{{/tr}} to:</p>
 {{/unless}}
-<form class="new-style" id="move_topic_form">
+<form id="move_topic_form">
+    <div class="move_topic_form_warning_container"></div>
     {{#if only_topic_edit }}
     <p>{{t "Rename topic to:" }}</p>
     {{else if from_message_actions_popover}}
diff --git a/web/tests/stream_topic_history.test.js b/web/tests/stream_topic_history.test.js
index 42b33b95a28333..888b978e92b98b 100644
--- a/web/tests/stream_topic_history.test.js
+++ b/web/tests/stream_topic_history.test.js
@@ -307,6 +307,25 @@ test("test_stream_has_topics", () => {
     assert.equal(stream_topic_history.stream_has_topics(stream_id), true);
 });
 
+test("test_stream_has_topic", () => {
+    const stream_id = 88;
+    const topic_name = "fake topic name";
+
+    assert.equal(stream_topic_history.stream_has_topic(stream_id, topic_name), false);
+
+    stream_topic_history.find_or_create(stream_id);
+
+    assert.equal(stream_topic_history.stream_has_topic(stream_id, topic_name), false);
+
+    stream_topic_history.add_message({
+        stream_id,
+        message_id: 888,
+        topic_name,
+    });
+
+    assert.equal(stream_topic_history.stream_has_topic(stream_id, topic_name), true);
+});
+
 test("server_history_end_to_end", () => {
     stream_topic_history.reset();