diff --git a/static/css/comment.css b/static/css/comment.css
index 4c86f730..68226ee5 100644
--- a/static/css/comment.css
+++ b/static/css/comment.css
@@ -221,4 +221,13 @@ input.error, textarea.error {
 /* OTHER */
 .hidden {
   display: none;
+}
+
+/* Disabled state for the Add Comment toolbar button */
+.addComment.disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+.addComment.disabled > a {
+  pointer-events: none;
 }
\ No newline at end of file
diff --git a/static/js/index.js b/static/js/index.js
index 4996cc46..ed65b598 100644
--- a/static/js/index.js
+++ b/static/js/index.js
@@ -143,9 +143,15 @@ EpComments.prototype.init = async function () {
   // `fromNow()` without needing to track the cached string anywhere.
   setInterval(() => this.refreshRelativeDates(), 60 * 1000);
 
+  // Cache the toolbar button element so it can be toggled efficiently in
+  // aceEditEvent (which fires on every keystroke / selection change).
+  this.$addCommentBtn = $('.addComment');
+  this.$addCommentBtnLink = this.$addCommentBtn.find('a');
+
   // On click comment icon toolbar
-  $('.addComment').on('click', (e) => {
+  this.$addCommentBtn.on('click', (e) => {
     e.preventDefault(); // stops focus from being lost
+    if (this.$addCommentBtn.hasClass('disabled')) return;
     this.displayNewCommentForm();
   });
 
@@ -1036,6 +1042,20 @@ EpComments.prototype.checkNoTextSelected = function (rep) {
   return noTextSelected;
 };
 
+// Enable or disable the "Add Comment" toolbar button based on whether text is selected.
+EpComments.prototype.updateAddCommentButtonState = function (hasSelection) {
+  const $btn = this.$addCommentBtn;
+  const $a = this.$addCommentBtnLink;
+  if (!$btn) return;
+  if (hasSelection) {
+    $btn.removeClass('disabled');
+    $a.removeAttr('aria-disabled');
+  } else {
+    $btn.addClass('disabled');
+    $a.attr('aria-disabled', 'true');
+  }
+};
+
 // Create form to add comment
 EpComments.prototype.createNewCommentFormIfDontExist = function (rep) {
   const data = this.getCommentData();
@@ -1295,6 +1315,9 @@ const hooks = {
     await Comments.initDone;
     pad.plugins.ep_comments_page = Comments;
 
+    // Start with the button disabled — no text is selected on load.
+    Comments.updateAddCommentButtonState(false);
+
     if (!$('#editorcontainerbox').hasClass('flex-layout')) {
       $.gritter.add({
         title: 'Error',
@@ -1343,6 +1366,13 @@ const hooks = {
           pad.plugins.ep_comments_page.shouldCollectComment = false;
         });
       }
+
+      // Update toolbar button enabled/disabled state based on whether text is selected.
+      const rep = context.rep;
+      if (rep) {
+        const ep = pad.plugins.ep_comments_page;
+        ep.updateAddCommentButtonState(!ep.checkNoTextSelected(rep));
+      }
     }
     return;
   },
diff --git a/static/tests/frontend-new/helper/comments.ts b/static/tests/frontend-new/helper/comments.ts
index 8d7122ac..8f9b8f70 100644
--- a/static/tests/frontend-new/helper/comments.ts
+++ b/static/tests/frontend-new/helper/comments.ts
@@ -91,7 +91,13 @@ export const selectLine = async (page: Page, lineIndex: number): Promise<void> =
 };
 
 // Clicks the toolbar Add Comment button (lives in the chrome page).
+// Waits for the button to be enabled (i.e., text is currently selected in the editor).
 export const clickAddCommentButton = async (page: Page): Promise<void> => {
+  await expect.poll(async () =>
+    page.locator('.addComment').first().evaluate(
+      (el: Element) => !el.classList.contains('disabled')
+    )
+  ).toBe(true);
   await page.locator('.addComment').first().click();
 };
 
diff --git a/static/tests/frontend-new/specs/newComment.spec.ts b/static/tests/frontend-new/specs/newComment.spec.ts
index d54ac09e..4844e927 100644
--- a/static/tests/frontend-new/specs/newComment.spec.ts
+++ b/static/tests/frontend-new/specs/newComment.spec.ts
@@ -1,6 +1,6 @@
 import {expect, test} from '@playwright/test';
 import {getPadBody} from 'ep_etherpad-lite/tests/frontend-new/helper/padHelper';
-import {aNewCommentsPad} from '../helper/comments';
+import {aNewCommentsPad, clickAddCommentButton} from '../helper/comments';
 
 test.describe('ep_comments_page - New comment', () => {
   test('new comment button focuses on comment textarea', async ({page}) => {
@@ -23,3 +23,83 @@ test.describe('ep_comments_page - New comment', () => {
     expect(isFocused).toBe(true);
   });
 });
+
+test.describe('ep_comments_page - Add Comment button disabled state', () => {
+  test.beforeEach(async ({page}) => {
+    // aNewCommentsPad waits for the plugin to initialise, which can take up to 60s.
+    test.setTimeout(60_000);
+  });
+
+  test('button is disabled on load (no text selected)', async ({page}) => {
+    await aNewCommentsPad(page);
+    await expect.poll(async () =>
+      page.locator('.addComment').first().evaluate(
+        (el: Element) => el.classList.contains('disabled')
+      )
+    ).toBe(true);
+    const ariaDisabled = await page.locator('.addComment a').first().getAttribute('aria-disabled');
+    expect(ariaDisabled).toBe('true');
+  });
+
+  test('button is enabled when text is selected', async ({page}) => {
+    await aNewCommentsPad(page);
+    const inner = await getPadBody(page);
+    await inner.click();
+    await page.keyboard.type('some text');
+    await inner.locator('div').first().click({clickCount: 3});
+    // Button should become enabled after selection.
+    await expect.poll(async () =>
+      page.locator('.addComment').first().evaluate(
+        (el: Element) => el.classList.contains('disabled')
+      )
+    ).toBe(false);
+    const ariaDisabled = await page.locator('.addComment a').first().getAttribute('aria-disabled');
+    expect(ariaDisabled).toBeNull();
+  });
+
+  test('button becomes disabled again after selection is cleared', async ({page}) => {
+    await aNewCommentsPad(page);
+    const inner = await getPadBody(page);
+    await inner.click();
+    await page.keyboard.type('some text');
+    // Select text.
+    await inner.locator('div').first().click({clickCount: 3});
+    await expect.poll(async () =>
+      page.locator('.addComment').first().evaluate(
+        (el: Element) => !el.classList.contains('disabled')
+      )
+    ).toBe(true);
+    // Click elsewhere to deselect.
+    await inner.click();
+    // Button should become disabled again.
+    await expect.poll(async () =>
+      page.locator('.addComment').first().evaluate(
+        (el: Element) => el.classList.contains('disabled')
+      )
+    ).toBe(true);
+  });
+
+  test('clicking disabled button does not open the comment form', async ({page}) => {
+    await aNewCommentsPad(page);
+    // Ensure no text is selected (button is disabled).
+    await expect.poll(async () =>
+      page.locator('.addComment').first().evaluate(
+        (el: Element) => el.classList.contains('disabled')
+      )
+    ).toBe(true);
+    // Click the <li> directly (bypassing pointer-events: none on the <a>).
+    await page.locator('.addComment').first().click();
+    // The comment form must not appear.
+    await expect(page.locator('#newComment.popup-show')).toHaveCount(0);
+  });
+
+  test('clicking enabled button opens the comment form', async ({page}) => {
+    await aNewCommentsPad(page);
+    const inner = await getPadBody(page);
+    await inner.click();
+    await page.keyboard.type('some text');
+    await inner.locator('div').first().click({clickCount: 3});
+    await clickAddCommentButton(page);
+    await expect(page.locator('#newComment.popup-show')).toHaveCount(1);
+  });
+});