diff --git a/options/locale/locale_en-US.json b/options/locale/locale_en-US.json
index 7cd1fa0245..940000eedd 100644
--- a/options/locale/locale_en-US.json
+++ b/options/locale/locale_en-US.json
@@ -1780,6 +1780,10 @@
   "repo.pulls.select_commit_hold_shift_for_range": "Select commit. Hold Shift and click to select a range.",
   "repo.pulls.review_only_possible_for_full_diff": "Review is only possible when viewing the full diff",
   "repo.pulls.filter_changes_by_commit": "Filter by commit",
+  "repo.pulls.filter_by_file_extension": "Filter by file extension",
+  "repo.pulls.select_all_file_extensions": "Select all",
+  "repo.pulls.deselect_all_file_extensions": "Deselect all",
+  "repo.pulls.apply_file_extension_filter": "Apply",
   "repo.pulls.nothing_to_compare": "These branches are equal. There is no need to create a pull request.",
   "repo.pulls.nothing_to_compare_have_tag": "The selected branches/tags are equal.",
   "repo.pulls.nothing_to_compare_and_allow_empty_pr": "These branches are equal. This PR will be empty.",
diff --git a/templates/repo/diff/box.tmpl b/templates/repo/diff/box.tmpl
index 390e41ec34..4649f9a45a 100644
--- a/templates/repo/diff/box.tmpl
+++ b/templates/repo/diff/box.tmpl
@@ -35,6 +35,12 @@
 			{{template "repo/diff/whitespace_dropdown" .}}
 			{{template "repo/diff/options_dropdown" .}}
 			{{if .PageIsPullFiles}}
+				<div id="diff-extension-filter" data-filter_by_file_extension="{{ctx.Locale.Tr "repo.pulls.filter_by_file_extension"}}" data-select_all="{{ctx.Locale.Tr "repo.pulls.select_all_file_extensions"}}" data-deselect_all="{{ctx.Locale.Tr "repo.pulls.deselect_all_file_extensions"}}" data-apply="{{ctx.Locale.Tr "repo.pulls.apply_file_extension_filter"}}">
+					{{/* the following will be replaced by vue component, but this avoids any loading artifacts till the vue component is initialized */}}
+					<div class="ui jump dropdown tiny basic button custom">
+						{{svg "octicon-filter"}}
+					</div>
+				</div>
 				<div id="diff-commit-select" data-merge-base="{{.MergeBase}}" data-issuelink="{{$.Issue.Link}}" data-queryparams="?style={{if $.IsSplitStyle}}split{{else}}unified{{end}}&whitespace={{$.WhitespaceBehavior}}&show-outdated={{$.ShowOutdatedComments}}" data-filter_changes_by_commit="{{ctx.Locale.Tr "repo.pulls.filter_changes_by_commit"}}">
 					{{/* the following will be replaced by vue component, but this avoids any loading artifacts till the vue component is initialized */}}
 					<div class="ui jump dropdown tiny basic button custom">
diff --git a/web_src/js/components/DiffFileExtensionFilter.vue b/web_src/js/components/DiffFileExtensionFilter.vue
new file mode 100644
index 0000000000..d2496d1f27
--- /dev/null
+++ b/web_src/js/components/DiffFileExtensionFilter.vue
@@ -0,0 +1,409 @@
+<script lang="ts" setup>
+import {ref, computed, onMounted, onUnmounted} from 'vue';
+import {SvgIcon} from '../svg.ts';
+import {generateElemId} from '../utils/dom.ts';
+
+/**
+ * Represents a file extension entry in the filter dropdown
+ * @property ext - Extension with dot (e.g., ".ts", ".go") or "(no extension)"
+ * @property checked - Whether this extension is currently selected for display
+ * @property count - Total number of diff files with this extension
+ */
+type Extension = {
+  ext: string,
+  checked: boolean,
+  count: number,
+}
+
+const el = document.querySelector<HTMLElement>('#diff-extension-filter')!;
+const menuVisible = ref(false);
+const extensions = ref<Array<Extension>>([]);
+const isFiltering = ref(false);
+const appliedExtensions = ref<Array<string> | null>(null);
+const searchQuery = ref('');
+const mutationObserver = ref<MutationObserver | null>(null);
+const uniqueIdMenu = generateElemId('diff-extension-filter-menu-');
+const locale = {
+  filter_by_file_extension: el.getAttribute('data-filter_by_file_extension') ?? 'Filter by extension',
+  select_all: el.getAttribute('data-select_all') ?? 'Select all',
+  deselect_all: el.getAttribute('data-deselect_all') ?? 'Deselect all',
+  apply: el.getAttribute('data-apply') ?? 'Apply',
+  search: el.getAttribute('data-search') ?? 'Search extensions...',
+} as Record<string, string>;
+
+/**
+ * Filter extensions based on search query
+ * Matches against extension name (e.g., ".ts", ".go")
+ */
+const filteredExtensions = computed(() => {
+  if (!searchQuery.value.trim()) {
+    return extensions.value;
+  }
+  const query = searchQuery.value.toLowerCase();
+  return extensions.value.filter((ext) => ext.ext.toLowerCase().includes(query));
+});
+
+/**
+ * Extract file extension from filename
+ * Returns the extension with dot (e.g., ".ts", ".go")
+ * Returns "(no extension)" for files without extension
+ */
+function getExtension(filename: string): string {
+  const lastDot = filename.lastIndexOf('.');
+  if (lastDot === -1 || lastDot === 0) {
+    return '(no extension)';
+  }
+  return filename.substring(lastDot);
+}
+
+/**
+ * Scan all diff-file-box elements and build extension list
+ * Checks current visibility state and sets checked state accordingly
+ * Updates the extensions array sorted by file count (descending)
+ */
+function scanExtensions() {
+  const extensionMap = new Map<string, {total: number, visible: number}>();
+  const fileBoxes = document.querySelectorAll<HTMLElement>('#diff-file-boxes .diff-file-box[data-new-filename]');
+
+  let hiddenCount = 0;
+  fileBoxes.forEach((box) => {
+    const filename = box.getAttribute('data-new-filename') || '';
+    const ext = getExtension(filename);
+    const isHidden = box.classList.contains('tw-hidden');
+    if (!extensionMap.has(ext)) {
+      extensionMap.set(ext, {total: 0, visible: 0});
+    }
+    const stats = extensionMap.get(ext)!;
+    stats.total += 1;
+    if (!isHidden) {
+      stats.visible += 1;
+    } else {
+      hiddenCount += 1;
+    }
+  });
+
+  extensions.value = Array.from(extensionMap.entries())
+    .map(([ext, stats]) => ({
+      ext,
+      checked: appliedExtensions.value ? appliedExtensions.value.includes(ext) : stats.visible > 0,
+      count: stats.total,
+    }))
+    .sort((a, b) => b.count - a.count);
+
+  isFiltering.value = hiddenCount > 0;
+}
+
+/**
+ * Apply filter to all diff file boxes by adding/removing tw-hidden class
+ * Updates isFiltering state and persists applied extensions for load-more sync
+ * @param checkedExtensions Set of extensions that should be visible
+ */
+function applyFilterToFileBoxes(checkedExtensions: Set<string>) {
+  const fileBoxes = document.querySelectorAll<HTMLElement>('#diff-file-boxes .diff-file-box[data-new-filename]');
+  let hiddenCount = 0;
+
+  fileBoxes.forEach((box) => {
+    const filename = box.getAttribute('data-new-filename') || '';
+    const ext = getExtension(filename);
+    const isChecked = checkedExtensions.has(ext);
+
+    if (isChecked) {
+      box.classList.remove('tw-hidden');
+    } else {
+      box.classList.add('tw-hidden');
+      hiddenCount += 1;
+    }
+  });
+
+  isFiltering.value = hiddenCount > 0;
+  appliedExtensions.value = hiddenCount > 0 ? Array.from(checkedExtensions) : null;
+}
+
+/**
+ * Focus a menu item element, updating tabIndex for keyboard navigation
+ * Focuses the first input or button within the element if available
+ * @param elem Element to focus
+ * @param prevElem Previous focused element to remove from tab order
+ */
+function focusElem(elem: HTMLElement | null, prevElem: HTMLElement | null) {
+  if (elem) {
+    elem.tabIndex = 0;
+    if (prevElem) prevElem.tabIndex = -1;
+    // Focus the input/button inside the menuitem if it exists, otherwise focus the item itself
+    const focusTarget = elem.querySelector('input, button') as HTMLElement || elem;
+    focusTarget.focus();
+  }
+}
+
+/**
+ * Toggle dropdown menu visibility
+ * Rescans extensions when opening, clears search when closing
+ */
+function toggleMenu() {
+  menuVisible.value = !menuVisible.value;
+  if (menuVisible.value) {
+    searchQuery.value = '';
+    scanExtensions();
+    setTimeout(() => {
+      const searchInput = el.querySelector('.diff-ext-search-input') as HTMLInputElement;
+      if (searchInput) searchInput.focus();
+    }, 0);
+  }
+}
+
+/**
+ * Select all file extensions
+ */
+function selectAll() {
+  for (const ext of extensions.value) {
+    ext.checked = true;
+  }
+}
+
+/**
+ * Deselect all file extensions
+ */
+function deselectAll() {
+  for (const ext of extensions.value) {
+    ext.checked = false;
+  }
+}
+
+/**
+ * Apply the current filter selection to diff files and close the dropdown
+ * Hides/shows diff-file-box elements based on checked extensions
+ */
+function applyFilter() {
+  const checkedExtensions = new Set(extensions.value.filter((e) => e.checked).map((e) => e.ext));
+  applyFilterToFileBoxes(checkedExtensions);
+  toggleMenu();
+}
+
+/**
+ * Close dropdown when clicking outside the component
+ * @param event Click event
+ */
+function onBodyClick(event: MouseEvent) {
+  if (!el.contains(event.target as Node)) {
+    if (menuVisible.value) {
+      toggleMenu();
+    }
+  }
+}
+
+/**
+ * Handle keyboard navigation within the dropdown menu
+ * Arrow Up/Down: navigate through checkboxes and buttons
+ * Space/Enter: toggle checkboxes or activate buttons
+ * Escape: close the dropdown
+ * @param event Keyboard event
+ */
+function onKeyDown(event: KeyboardEvent) {
+  if (!menuVisible.value) return;
+  const currentFocused = document.activeElement as HTMLElement;
+  if (!el.contains(currentFocused)) return;
+
+  const menu = el.querySelector('.menu') as HTMLElement;
+  const focusableItems = Array.from(menu.querySelectorAll('[role="menuitem"]')) as HTMLElement[];
+
+  if (!focusableItems.length) return;
+
+  const currentIndex = focusableItems.indexOf(currentFocused.closest('[role="menuitem"]') as HTMLElement);
+
+  switch (event.key) {
+    case 'ArrowDown': {
+      event.preventDefault();
+      const nextIndex = currentIndex === -1 ? 0 : Math.min(currentIndex + 1, focusableItems.length - 1);
+      focusElem(focusableItems[nextIndex], currentIndex >= 0 ? focusableItems[currentIndex] : null);
+      break;
+    }
+    case 'ArrowUp': {
+      event.preventDefault();
+      const prevIndex = currentIndex === -1 ? focusableItems.length - 1 : Math.max(currentIndex - 1, 0);
+      focusElem(focusableItems[prevIndex], currentIndex >= 0 ? focusableItems[currentIndex] : null);
+      break;
+    }
+    case ' ':
+    case 'Enter': {
+      event.preventDefault();
+      const currentElement = document.activeElement as HTMLElement;
+
+      // Try to find and toggle a checkbox (currentElement may be the input itself or a parent)
+      const checkbox = (currentElement?.matches('input[type="checkbox"]')
+        ? currentElement
+        : currentElement?.querySelector('input[type="checkbox"]')) as HTMLInputElement | null;
+      if (checkbox) {
+        checkbox.checked = !checkbox.checked;
+        checkbox.dispatchEvent(new Event('change', {bubbles: true}));
+        break;
+      }
+
+      // If focused element is a button, click it
+      if (currentElement?.tagName === 'BUTTON') {
+        currentElement.click();
+      }
+      break;
+    }
+    case 'Escape':
+      event.preventDefault();
+      if (currentIndex >= 0) {
+        focusableItems[currentIndex].tabIndex = -1;
+      }
+      toggleMenu();
+      break;
+  }
+}
+
+onMounted(() => {
+  document.body.addEventListener('click', onBodyClick, true);
+  el.addEventListener('keydown', onKeyDown);
+
+  // Watch for new files being added (e.g., when "load more" is clicked)
+  const fileBoxesContainer = document.querySelector('#diff-file-boxes');
+  if (fileBoxesContainer) {
+    mutationObserver.value = new MutationObserver(() => {
+      if (appliedExtensions.value) {
+        applyFilterToFileBoxes(new Set(appliedExtensions.value));
+      }
+
+      if (menuVisible.value) {
+        scanExtensions();
+      }
+    });
+    mutationObserver.value.observe(fileBoxesContainer, {childList: true, subtree: false});
+  }
+});
+
+onUnmounted(() => {
+  document.body.removeEventListener('click', onBodyClick, true);
+  el.removeEventListener('keydown', onKeyDown);
+
+  if (mutationObserver.value) {
+    mutationObserver.value.disconnect();
+  }
+});
+</script>
+<template>
+  <div class="ui scrolling dropdown custom diff-file-extension-filter">
+    <button
+      class="ui tiny basic button"
+      :class="{'diff-ext-filter-btn-active': isFiltering}"
+      @click="toggleMenu()"
+      :data-tooltip-content="locale.filter_by_file_extension"
+      aria-haspopup="true"
+      :aria-label="locale.filter_by_file_extension"
+      :aria-controls="uniqueIdMenu"
+    >
+      <SvgIcon name="octicon-filter"/>
+    </button>
+    <!-- this dropdown is not managed by Fomantic UI, so it needs some classes like "transition" explicitly -->
+    <div class="left menu transition" :id="uniqueIdMenu" :class="{visible: menuVisible}" v-show="menuVisible" v-cloak :aria-expanded="menuVisible ? 'true': 'false'">
+      <div class="header">{{ locale.filter_by_file_extension }}</div>
+      <div class="ui divider tw-mt-2 tw-mb-0"/>
+      <!-- Search input -->
+      <div class="ui form tw-mb-2">
+        <div class="ui input fluid field tw-mb-0">
+          <input
+            type="text"
+            v-model="searchQuery"
+            class="diff-ext-search-input"
+            :placeholder="locale.search"
+            @keydown.escape="toggleMenu()"
+          >
+        </div>
+      </div>
+      <div class="ui divider tw-mt-2 tw-mb-0"/>
+      <div class="ui form">
+        <!-- Extension checkboxes -->
+        <div class="grouped fields">
+          <div v-if="filteredExtensions.length > 0">
+            <template v-for="ext in filteredExtensions" :key="ext.ext">
+              <div class="field" role="menuitem" tabindex="-1">
+                <div class="ui checkbox">
+                  <input
+                    type="checkbox"
+                    :id="`ext-filter-${ext.ext}`"
+                    v-model="ext.checked"
+                  >
+                  <label :for="`ext-filter-${ext.ext}`" class="tw-cursor-pointer">
+                    <span class="tw-font-mono">{{ ext.ext }}</span>
+                    <span class="tw-text-text-light-2"> ({{ ext.count }})</span>
+                  </label>
+                </div>
+              </div>
+            </template>
+          </div>
+          <div v-if="filteredExtensions.length === 0" class="tw-py-4 tw-text-center tw-text-text-light-2">
+            {{ locale.no_results ?? 'No extensions found' }}
+          </div>
+        </div>
+      </div>
+
+      <!-- Select all / Deselect all buttons -->
+      <div class="ui divider tw-my-2"/>
+      <div class="tw-flex tw-items-center tw-justify-center tw-gap-4 tw-px-2 tw-py-1">
+        <button type="button" class="diff-ext-text-btn" tabindex="-1" role="menuitem" @click="selectAll()">{{ locale.select_all }}</button>
+        <button type="button" class="diff-ext-text-btn" tabindex="-1" role="menuitem" @click="deselectAll()">{{ locale.deselect_all }}</button>
+      </div>
+
+      <!-- Apply button -->
+      <div class="ui divider tw-my-2"/>
+      <button type="button" class="ui button fluid" tabindex="-1" role="menuitem" @click="applyFilter()">
+        {{ locale.apply }}
+      </button>
+    </div>
+  </div>
+</template>
+<style scoped>
+  .ui.dropdown.diff-file-extension-filter .menu {
+    margin-top: 0.25em;
+    overflow-x: hidden;
+    max-height: 450px;
+    padding: 0.75rem;
+    padding-top: 0.5rem;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .menu > .header {
+    margin-top: 0;
+    padding-top: 0;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .menu .ui.form {
+    margin: 0;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .grouped.fields > div > .field {
+    margin-bottom: 0.5rem;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .grouped.fields > div > .field:last-child {
+    margin-bottom: 0;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .diff-ext-filter-btn-active {
+    outline: 1px solid var(--color-primary);
+    outline-offset: -1px;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .diff-ext-text-btn {
+    background: none;
+    border: none;
+    padding: 0;
+    color: var(--color-primary);
+    cursor: pointer;
+    font-size: inherit;
+    text-align: center;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .diff-ext-text-btn:hover {
+    text-decoration: underline;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .diff-ext-search-input {
+    width: 100%;
+  }
+
+  .ui.dropdown.diff-file-extension-filter .ui.input {
+    margin-bottom: 0.5rem;
+  }
+</style>
diff --git a/web_src/js/features/repo-diff-extensionfilter.ts b/web_src/js/features/repo-diff-extensionfilter.ts
new file mode 100644
index 0000000000..7d1f735f9a
--- /dev/null
+++ b/web_src/js/features/repo-diff-extensionfilter.ts
@@ -0,0 +1,10 @@
+import {createApp} from 'vue';
+import DiffFileExtensionFilter from '../components/DiffFileExtensionFilter.vue';
+
+export function initDiffFileExtensionFilter() {
+  const el = document.querySelector('#diff-extension-filter');
+  if (!el) return;
+
+  const extensionFilter = createApp(DiffFileExtensionFilter);
+  extensionFilter.mount(el);
+}
diff --git a/web_src/js/features/repo-diff.ts b/web_src/js/features/repo-diff.ts
index 0c8b1357b0..4b420f79d5 100644
--- a/web_src/js/features/repo-diff.ts
+++ b/web_src/js/features/repo-diff.ts
@@ -1,6 +1,7 @@
 import {initRepoIssueContentHistory} from './repo-issue-content.ts';
 import {initDiffFileTree} from './repo-diff-filetree.ts';
 import {initDiffCommitSelect} from './repo-diff-commitselect.ts';
+import {initDiffFileExtensionFilter} from './repo-diff-extensionfilter.ts';
 import {validateTextareaNonEmpty} from './comp/ComboMarkdownEditor.ts';
 import {initViewedCheckboxListenerFor, initExpandAndCollapseFilesButton} from './pull-view-file.ts';
 import {initImageDiff} from './imagediff.ts';
@@ -281,6 +282,7 @@ export function initRepoDiffView() {
   initRepoDiffConversationNav(); // "previous" and "next" buttons only appear on "diff" page
   initDiffFileTree();
   initDiffCommitSelect();
+  initDiffFileExtensionFilter();
   initRepoDiffShowMore();
   initDiffHeaderPopup();
   initViewedCheckboxListenerFor();