diff --git a/docs/changelog.md b/docs/changelog.md
index 661161c6..1d8531a8 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -12,6 +12,13 @@ and this project adheres to the
 [Python Version Specification](https://packaging.python.org/en/latest/specifications/version-specifiers/).
 See the [Contributing Guide](contributing.md) for details.
 
+## [unreleased]
+
+### Fixed
+
+* Treat `picture` as a block-level HTML element so comments inside it do not
+  split the element into invalid paragraphs (#1643).
+
 ## [3.11.0] - 2026-09-25
 
 ### Changed
diff --git a/docs/extensions/md_in_html.md b/docs/extensions/md_in_html.md
index 13a9a88b..6181f1dc 100644
--- a/docs/extensions/md_in_html.md
+++ b/docs/extensions/md_in_html.md
@@ -45,8 +45,8 @@ When the `markdown` attribute is set to `"1"`, then the parser will use the defa
 
 The following tags have the `block` behavior by default: `article`, `aside`, `blockquote`, `body`, `colgroup`,
 `details`, `div`, `dl`, `fieldset`, `figcaption`, `figure`, `footer`, `form`, `group`, `header`, `hgroup`, `hr`,
-`iframe`,  `main`, `map`, `menu`, `nav`, `noscript`, `object`, `ol`, `output`, `progress`, `section`, `table`,
-`tbody`, `tfoot`, `thead`, `tr`,  `ul` and `video`.
+`iframe`,  `main`, `map`, `menu`, `nav`, `noscript`, `object`, `ol`, `output`, `picture`, `progress`, `section`,
+`table`, `tbody`, `tfoot`, `thead`, `tr`,  `ul` and `video`.
 
 For example:
 
diff --git a/markdown/util.py b/markdown/util.py
index 29ffa247..439e66dd 100644
--- a/markdown/util.py
+++ b/markdown/util.py
@@ -53,7 +53,7 @@
     'p', 'pre', 'section', 'table', 'ul',
     # Other elements which Markdown should not be mucking up the contents of.
     'canvas', 'colgroup', 'dd', 'body', 'dt', 'group', 'html', 'iframe', 'li', 'legend',
-    'math', 'map', 'noscript', 'output', 'object', 'option', 'progress', 'script',
+    'math', 'map', 'noscript', 'output', 'object', 'option', 'picture', 'progress', 'script',
     'style', 'summary', 'tbody', 'td', 'textarea', 'tfoot', 'th', 'thead', 'tr', 'video',
     'center'
 ]
diff --git a/tests/test_syntax/blocks/test_html_blocks.py b/tests/test_syntax/blocks/test_html_blocks.py
index ecc10f45..52da2a10 100644
--- a/tests/test_syntax/blocks/test_html_blocks.py
+++ b/tests/test_syntax/blocks/test_html_blocks.py
@@ -1004,6 +1004,35 @@ def test_raw_comment_nested(self):
             )
         )
 
+    def test_raw_picture_with_comment(self):
+        # HTML comments inside <picture> must not split the element (see #1643).
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                <picture>
+                  <!-- comment -->
+                  <source media="(prefers-color-scheme: dark)" srcset="dark.svg">
+                  <img alt="Example" src="example.svg">
+                </picture>
+                """
+            ),
+            self.dedent(
+                """
+                <picture>
+                  <!-- comment -->
+                  <source media="(prefers-color-scheme: dark)" srcset="dark.svg">
+                  <img alt="Example" src="example.svg">
+                </picture>
+                """
+            )
+        )
+
+    def test_inline_picture_remains_inline(self):
+        self.assertMarkdownRenders(
+            'See <picture><img alt="Example" src="x.svg"></picture> here.',
+            '<p>See <picture><img alt="Example" src="x.svg"></picture> here.</p>'
+        )
+
     def test_comment_in_code_block(self):
         self.assertMarkdownRenders(
             '    <!-- *foo* -->',