diff --git a/assets/l10n/app_en.arb b/assets/l10n/app_en.arb
index edb01af23d..95b924fa18 100644
--- a/assets/l10n/app_en.arb
+++ b/assets/l10n/app_en.arb
@@ -366,6 +366,10 @@
   "@serverUrlValidationErrorUnsupportedScheme": {
     "description": "Error message when URL has an unsupported scheme."
   },
+  "spoilerDefaultHeaderText": "Spoiler",
+  "@spoilerDefaultHeaderText": {
+    "description": "The default header text in a spoiler block ( https://zulip.com/help/spoilers )."
+  },
   "markAllAsReadLabel": "Mark all messages as read",
   "@markAllAsReadLabel": {
     "description": "Button text to mark messages as read."
diff --git a/lib/model/content.dart b/lib/model/content.dart
index 1aba6f895d..05067c1d58 100644
--- a/lib/model/content.dart
+++ b/lib/model/content.dart
@@ -258,6 +258,21 @@ class QuotationNode extends BlockContentNode {
   }
 }
 
+class SpoilerNode extends BlockContentNode {
+  const SpoilerNode({super.debugHtmlNode, required this.header, required this.content});
+
+  final List<BlockContentNode> header;
+  final List<BlockContentNode> content;
+
+  @override
+  List<DiagnosticsNode> debugDescribeChildren() {
+    return [
+      _BlockContentListNode(header).toDiagnosticsNode(name: 'header'),
+      _BlockContentListNode(content).toDiagnosticsNode(name: 'content'),
+    ];
+  }
+}
+
 class CodeBlockNode extends BlockContentNode {
   const CodeBlockNode(this.spans, {super.debugHtmlNode});
 
@@ -809,6 +824,26 @@ class _ZulipContentParser {
     return ListNode(listStyle!, items, debugHtmlNode: debugHtmlNode);
   }
 
+  BlockContentNode parseSpoilerNode(dom.Element divElement) {
+    assert(_debugParserContext == _ParserContext.block);
+    assert(divElement.localName == 'div'
+        && divElement.className == 'spoiler-block');
+
+    if (divElement.nodes case [
+      dom.Element(
+        localName: 'div', className: 'spoiler-header', nodes: var headerNodes),
+      dom.Element(
+        localName: 'div', className: 'spoiler-content', nodes: var contentNodes),
+    ]) {
+      return SpoilerNode(
+        header: parseBlockContentList(headerNodes),
+        content: parseBlockContentList(contentNodes),
+      );
+    } else {
+      return UnimplementedBlockContentNode(htmlNode: divElement);
+    }
+  }
+
   BlockContentNode parseCodeBlock(dom.Element divElement) {
     assert(_debugParserContext == _ParserContext.block);
     final mainElement = () {
@@ -977,6 +1012,10 @@ class _ZulipContentParser {
         parseBlockContentList(element.nodes));
     }
 
+    if (localName == 'div' && className == 'spoiler-block') {
+      return parseSpoilerNode(element);
+    }
+
     if (localName == 'div' && className == 'codehilite') {
       return parseCodeBlock(element);
     }
diff --git a/lib/widgets/content.dart b/lib/widgets/content.dart
index 1f70556f86..1c5d23de87 100644
--- a/lib/widgets/content.dart
+++ b/lib/widgets/content.dart
@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:intl/intl.dart';
+import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
 import '../api/core.dart';
 import '../api/model/model.dart';
@@ -79,6 +80,8 @@ class BlockContentList extends StatelessWidget {
           return Quotation(node: node);
         } else if (node is ListNode) {
           return ListNodeWidget(node: node);
+        } else if (node is SpoilerNode) {
+          return Spoiler(node: node);
         } else if (node is CodeBlockNode) {
           return CodeBlock(node: node);
         } else if (node is MathBlockNode) {
@@ -235,6 +238,93 @@ class ListItemWidget extends StatelessWidget {
   }
 }
 
+class Spoiler extends StatefulWidget {
+  const Spoiler({super.key, required this.node});
+
+  final SpoilerNode node;
+
+  @override
+  State<Spoiler> createState() => _SpoilerState();
+}
+
+class _SpoilerState extends State<Spoiler> with TickerProviderStateMixin {
+  bool expanded = false;
+
+  late final AnimationController _controller = AnimationController(
+    duration: const Duration(milliseconds: 400), vsync: this);
+  late final Animation<double> _animation = CurvedAnimation(
+    parent: _controller, curve: Curves.easeInOut);
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  void _handleTap() {
+    setState(() {
+      if (!expanded) {
+        _controller.forward();
+        expanded = true;
+      } else {
+        _controller.reverse();
+        expanded = false;
+      }
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final zulipLocalizations = ZulipLocalizations.of(context);
+    final header = widget.node.header;
+    final effectiveHeader = header.isNotEmpty
+      ? header
+      : [ParagraphNode(links: null,
+           nodes: [TextNode(zulipLocalizations.spoilerDefaultHeaderText)])];
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(0, 5, 0, 15),
+      child: DecoratedBox(
+        decoration: BoxDecoration(
+          border: Border.all(color: const Color(0xff808080)),
+          borderRadius: BorderRadius.circular(10),
+        ),
+        child: Padding(padding: const EdgeInsetsDirectional.fromSTEB(10, 2, 8, 2),
+          child: Column(
+            children: [
+              GestureDetector(
+                behavior: HitTestBehavior.translucent,
+                onTap: _handleTap,
+                child: Padding(
+                  padding: const EdgeInsets.all(5),
+                  child: Row(crossAxisAlignment: CrossAxisAlignment.end, children: [
+                    Expanded(
+                      child: DefaultTextStyle.merge(
+                        style: weightVariableTextStyle(context, wght: 700),
+                        child: BlockContentList(
+                          nodes: effectiveHeader))),
+                    RotationTransition(
+                      turns: _animation.drive(Tween(begin: 0, end: 0.5)),
+                      child: const Icon(color: Color(0xffd4d4d4), size: 25,
+                        Icons.expand_more)),
+                  ]))),
+              FadeTransition(
+                opacity: _animation,
+                child: const SizedBox(height: 0, width: double.infinity,
+                  child: DecoratedBox(
+                    decoration: BoxDecoration(
+                      border: Border(
+                        bottom: BorderSide(width: 1, color: Color(0xff808080))))))),
+              SizeTransition(
+                sizeFactor: _animation,
+                axis: Axis.vertical,
+                axisAlignment: -1,
+                child: Padding(
+                  padding: const EdgeInsets.all(5),
+                  child: BlockContentList(nodes: widget.node.content))),
+            ]))));
+  }
+}
+
 class MessageImageList extends StatelessWidget {
   const MessageImageList({super.key, required this.node});
 
diff --git a/test/model/content_test.dart b/test/model/content_test.dart
index 572e176157..8607928c5e 100644
--- a/test/model/content_test.dart
+++ b/test/model/content_test.dart
@@ -125,6 +125,78 @@ class ContentExample {
     const ImageEmojiNode(
       src: '/static/generated/emoji/images/emoji/unicode/zulip.png', alt: ':zulip:'));
 
+  static const spoilerDefaultHeader = ContentExample(
+    'spoiler with default header',
+    '```spoiler\nhello world\n```',
+    expectedText: 'Spoiler', // or a translation
+    '<div class="spoiler-block"><div class="spoiler-header">\n'
+        '</div><div class="spoiler-content" aria-hidden="true">\n'
+        '<p>hello world</p>\n'
+        '</div></div>',
+    [SpoilerNode(
+      header: [],
+      content: [ParagraphNode(links: null, nodes: [TextNode('hello world')])],
+    )]);
+
+  static const spoilerPlainCustomHeader = ContentExample(
+    'spoiler with plain custom header',
+    '```spoiler hello\nworld\n```',
+    expectedText: 'hello',
+    '<div class="spoiler-block"><div class="spoiler-header">\n'
+        '<p>hello</p>\n'
+        '</div><div class="spoiler-content" aria-hidden="true">\n'
+        '<p>world</p>\n'
+        '</div></div>',
+    [SpoilerNode(
+      header: [ParagraphNode(links: null, nodes: [TextNode('hello')])],
+      content: [ParagraphNode(links: null, nodes: [TextNode('world')])],
+    )]);
+
+  static const spoilerRichHeaderAndContent = ContentExample(
+    'spoiler with rich header and content',
+    '```spoiler 1. * ## hello\n*italic* [zulip](https://zulip.com/)\n```',
+    expectedText: 'hello',
+    '<div class="spoiler-block"><div class="spoiler-header">\n'
+        '<ol>\n<li>\n<ul>\n<li>\n<h2>hello</h2>\n</li>\n</ul>\n</li>\n</ol>\n</div>'
+        '<div class="spoiler-content" aria-hidden="true">\n'
+        '<p><em>italic</em> <a href="https://zulip.com/">zulip</a></p>\n'
+        '</div></div>',
+    [SpoilerNode(
+      header: [ListNode(ListStyle.ordered, [
+          [ListNode(ListStyle.unordered, [
+            [HeadingNode(level: HeadingLevel.h2, links: null, nodes: [
+              TextNode('hello'),
+            ])]
+          ])],
+        ])],
+      content: [ParagraphNode(links: null, nodes: [
+          EmphasisNode(nodes: [TextNode('italic')]),
+          TextNode(' '),
+          LinkNode(url: 'https://zulip.com/', nodes: [TextNode('zulip')])
+        ])],
+    )]);
+
+  static const spoilerHeaderHasImage = ContentExample(
+    'spoiler a header that has an image in it',
+    '```spoiler [image](https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3)\nhello world\n```',
+    '<div class="spoiler-block"><div class="spoiler-header">\n'
+      '<p><a href="https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3">image</a></p>\n'
+      '<div class="message_inline_image"><a href="https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3" title="image"><img src="https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3"></a></div></div><div class="spoiler-content" aria-hidden="true">\n'
+      '<p>hello world</p>\n'
+      '</div></div>\n',
+    [SpoilerNode(
+      header: [
+        ParagraphNode(links: null, nodes: [
+          LinkNode(url: 'https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3',
+            nodes: [TextNode('image')]),
+        ]),
+        ImageNodeList([
+          ImageNode(srcUrl: 'https://chat.zulip.org/user_avatars/2/realm/icon.png?version=3'),
+        ]),
+      ],
+      content: [ParagraphNode(links: null, nodes: [TextNode('hello world')])],
+    )]);
+
   static const quotation = ContentExample(
     'quotation',
     "```quote\nwords\n```",
@@ -726,6 +798,11 @@ void main() {
       ]);
   });
 
+  testParseExample(ContentExample.spoilerDefaultHeader);
+  testParseExample(ContentExample.spoilerPlainCustomHeader);
+  testParseExample(ContentExample.spoilerRichHeaderAndContent);
+  testParseExample(ContentExample.spoilerHeaderHasImage);
+
   group('track links inside block-inline containers', () {
     testParse('multiple links in paragraph',
       // "before[text](/there)mid[other](/else)after"
diff --git a/test/widgets/content_test.dart b/test/widgets/content_test.dart
index 7653dcb55f..d115580127 100644
--- a/test/widgets/content_test.dart
+++ b/test/widgets/content_test.dart
@@ -15,6 +15,7 @@ import 'package:zulip/widgets/page.dart';
 import 'package:zulip/widgets/store.dart';
 
 import '../example_data.dart' as eg;
+import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../model/content_test.dart';
 import '../model/test_store.dart';
@@ -93,6 +94,97 @@ void main() {
     });
   });
 
+  group('Spoiler', () {
+    testContentSmoke(ContentExample.spoilerDefaultHeader);
+    testContentSmoke(ContentExample.spoilerPlainCustomHeader);
+    testContentSmoke(ContentExample.spoilerRichHeaderAndContent);
+
+    group('interactions: spoiler with tappable content (an image) in the header', () {
+      Future<List<Route<dynamic>>> prepareContent(WidgetTester tester, String html) async {
+        addTearDown(testBinding.reset);
+        await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+        prepareBoringImageHttpClient();
+
+        final pushedRoutes = <Route<dynamic>>[];
+        final testNavObserver = TestNavigatorObserver()
+          ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
+
+        await tester.pumpWidget(GlobalStoreWidget(child: MaterialApp(
+          localizationsDelegates: ZulipLocalizations.localizationsDelegates,
+          supportedLocales: ZulipLocalizations.supportedLocales,
+          navigatorObservers: [testNavObserver],
+          home: PerAccountStoreWidget(accountId: eg.selfAccount.id,
+            child: MessageContent(
+              message: eg.streamMessage(content: html),
+              content: parseContent(html))))));
+        await tester.pump(); // global store
+        await tester.pump(); // per-account store
+        debugNetworkImageHttpClientProvider = null;
+
+        // `tester.pumpWidget` introduces an initial route;
+        // remove it so consumers only have newly pushed routes.
+        assert(pushedRoutes.length == 1);
+        pushedRoutes.removeLast();
+        return pushedRoutes;
+      }
+
+      void checkIsExpanded(WidgetTester tester,
+        bool expected, {
+        Finder? contentFinder,
+      }) {
+        final sizeTransition = tester.widget<SizeTransition>(find.ancestor(
+          of: contentFinder ?? find.text('hello world'),
+          matching: find.byType(SizeTransition),
+        ));
+        check(sizeTransition.sizeFactor)
+          ..value.equals(expected ? 1 : 0)
+          ..status.equals(expected ? AnimationStatus.completed : AnimationStatus.dismissed);
+      }
+
+      const example = ContentExample.spoilerHeaderHasImage;
+
+      testWidgets('tap image', (tester) async {
+        final pushedRoutes = await prepareContent(tester, example.html);
+
+        await tester.tapAt(tester.getCenter(find.byType(RealmContentNetworkImage)));
+        check(pushedRoutes).single.isA<AccountPageRouteBuilder>()
+          .fullscreenDialog.isTrue(); // recognize the lightbox
+      });
+
+      testWidgets('tap header on expand/collapse icon', (tester) async {
+        final pushedRoutes = await prepareContent(tester, example.html);
+        checkIsExpanded(tester, false);
+
+        await tester.tap(find.byIcon(Icons.expand_more));
+        await tester.pumpAndSettle();
+        check(pushedRoutes).isEmpty(); // no lightbox
+        checkIsExpanded(tester, true);
+
+        await tester.tap(find.byIcon(Icons.expand_more));
+        await tester.pumpAndSettle();
+        check(pushedRoutes).isEmpty(); // no lightbox
+        checkIsExpanded(tester, false);
+      });
+
+      testWidgets('tap header away from expand/collapse icon (and image)', (tester) async {
+        final pushedRoutes = await prepareContent(tester, example.html);
+        checkIsExpanded(tester, false);
+
+        await tester.tapAt(
+          tester.getTopRight(find.byType(RealmContentNetworkImage)) + const Offset(10, 0));
+        await tester.pumpAndSettle();
+        check(pushedRoutes).isEmpty(); // no lightbox
+        checkIsExpanded(tester, true);
+
+        await tester.tapAt(
+          tester.getTopRight(find.byType(RealmContentNetworkImage)) + const Offset(10, 0));
+        await tester.pumpAndSettle();
+        check(pushedRoutes).isEmpty(); // no lightbox
+        checkIsExpanded(tester, false);
+      });
+    });
+  });
+
   testContentSmoke(ContentExample.quotation);
 
   group('MessageImage, MessageImageList', () {
diff --git a/test/widgets/page_checks.dart b/test/widgets/page_checks.dart
index f6effa2606..1ce597033a 100644
--- a/test/widgets/page_checks.dart
+++ b/test/widgets/page_checks.dart
@@ -9,3 +9,7 @@ extension WidgetRouteChecks on Subject<WidgetRoute> {
 extension AccountPageRouteMixinChecks on Subject<AccountPageRouteMixin> {
   Subject<int> get accountId => has((x) => x.accountId, 'accountId');
 }
+
+extension PageRouteChecks on Subject<PageRoute> {
+  Subject<bool> get fullscreenDialog => has((x) => x.fullscreenDialog, 'fullscreenDialog');
+}