diff --git a/build/__snapshots__/features-meta.json b/build/__snapshots__/features-meta.json
index 0a917306e13c..cfe9e23236d5 100644
--- a/build/__snapshots__/features-meta.json
+++ b/build/__snapshots__/features-meta.json
@@ -234,6 +234,12 @@
 		"css": true,
 		"screenshot": "https://user-images.githubusercontent.com/1402241/220607557-f8ea0863-f05b-48c8-a447-1fec42af0afd.gif"
 	},
+	{
+		"id": "dim-viewed-files",
+		"description": "Dims viewed files and folders containing only viewed files in the PR file tree, keeping unreviewed files easy to find.",
+		"css": true,
+		"screenshot": "https://private-user-images.githubusercontent.com/9824294/665923384-1b7a5682-ec3d-4ca4-8608-71ad3b6a2228.png"
+	},
 	{
 		"id": "download-folder-button",
 		"description": "Adds a button to download entire folders, via https://download-directory.github.io.",
diff --git a/build/__snapshots__/imported-features.json b/build/__snapshots__/imported-features.json
index 90e048285e5c..1a5528bd9c37 100644
--- a/build/__snapshots__/imported-features.json
+++ b/build/__snapshots__/imported-features.json
@@ -39,6 +39,7 @@
 	"default-branch-button",
 	"delete-branch",
 	"dim-bots",
+	"dim-viewed-files",
 	"download-folder-button",
 	"easy-toggle-commit-messages",
 	"easy-toggle-files",
diff --git a/readme.md b/readme.md
index f27fd3682b6a..3f559fe9f27c 100644
--- a/readme.md
+++ b/readme.md
@@ -242,6 +242,7 @@ https://github.com/refined-github/refined-github/wiki/Contributing#metadata-guid
 - [](# "clean-checks-list") Prioritizes failing and pending checks in the merge checks list, and untruncates long check names.
 - [](# "mobile-tabs-pr") Makes the PR tabs smaller so they fit in the screen without scrolling.
 - [](# "linkify-pr-review-state") [Linkifies the PR review state labels ("Draft", "Changes requested", etc) in PR lists to filter by that state.](https://github.com/user-attachments/assets/9c59ba75-d2f2-41d4-a214-24d123f78428)
+- [](# "dim-viewed-files") [Dims viewed files and folders containing only viewed files in the PR file tree, keeping unreviewed files easy to find.](https://private-user-images.githubusercontent.com/9824294/665923384-1b7a5682-ec3d-4ca4-8608-71ad3b6a2228.png)
 
 <!--
 Refer to style guide in the wiki. Keep this message between sections.
diff --git a/source/features/dim-viewed-files.css b/source/features/dim-viewed-files.css
new file mode 100644
index 000000000000..d377fb37254c
--- /dev/null
+++ b/source/features/dim-viewed-files.css
@@ -0,0 +1,18 @@
+/* Files: rows marked as viewed by the JS */
+.rgh-dim-viewed-files:not(:hover, :focus-within) {
+	opacity: 50%;
+}
+
+/* Folders: dim only the folder's own row (not its subtree, to avoid compounding opacity) when it contains viewed files and no unviewed ones */
+ul[aria-label='File Tree'] {
+	[role='treeitem'][aria-expanded] {
+		&:has(.rgh-dim-viewed-files):not(
+			:has(
+				[role='treeitem']:not([aria-expanded], .rgh-dim-viewed-files)
+			)
+		)
+			> div:not(:hover, :focus-within) {
+			opacity: 50%;
+		}
+	}
+}
diff --git a/source/features/dim-viewed-files.tsx b/source/features/dim-viewed-files.tsx
new file mode 100644
index 000000000000..4d151c4aa795
--- /dev/null
+++ b/source/features/dim-viewed-files.tsx
@@ -0,0 +1,51 @@
+import './dim-viewed-files.css';
+
+import debounce from 'debounce-fn';
+import delegate from 'delegate-it';
+import * as pageDetect from 'github-url-detection';
+import {$$optional, closestElement, elementExists} from 'select-dom';
+
+import features from '../feature-manager.js';
+import observe from '../helpers/selector-observer.js';
+
+const viewedToggleSelector =
+	':is(button[class*="MarkAsViewedButton"], [class*="diff-file-header"] button[aria-pressed])';
+const treeLinkSelector = 'ul[aria-label="File Tree"] a[href*="#"]';
+const dimmedClass = 'rgh-dim-viewed-files';
+
+const updateTree = debounce((): void => {
+	for (const link of $$optional(treeLinkSelector)) {
+		const row = closestElement('[role="treeitem"]', link);
+		const viewed = elementExists(
+			`#${CSS.escape(link.hash.slice(1))} ${viewedToggleSelector}[aria-pressed="true"]`,
+		);
+		row.classList.toggle(dimmedClass, viewed);
+	}
+}, {wait: 100});
+
+export function init(signal: AbortSignal): void {
+	observe(
+		[
+			treeLinkSelector,
+			// The pressed selector also catches viewed state loaded after the button renders.
+			`${viewedToggleSelector}[aria-pressed="true"]`,
+		],
+		updateTree,
+		{signal},
+	);
+	delegate(viewedToggleSelector, 'click', updateTree, {signal});
+}
+
+void features.add(import.meta.url, {
+	include: [pageDetect.isPRFiles],
+	exclude: [pageDetect.isPRFile404, pageDetect.isPRCommit],
+	init,
+});
+
+/*
+
+Test URLs
+
+https://github.com/refined-github/sandbox/pull/55/files
+
+*/
diff --git a/source/refined-github.ts b/source/refined-github.ts
index a5be26a722d1..4e2a0e14be7a 100644
--- a/source/refined-github.ts
+++ b/source/refined-github.ts
@@ -221,4 +221,5 @@ import './features/delete-branch.js';
 import './features/pr-from-branch.js';
 import './features/mark-locked.js';
 import './features/subscribe-releases.js';
+import './features/dim-viewed-files.js';
 import './features/absolute-time.js';