diff --git a/.spell-dict b/.spell-dict
index c82a11dd..c0f0c880 100644
--- a/.spell-dict
+++ b/.spell-dict
@@ -159,6 +159,7 @@ unescaping
 unittest
 unordered
 unsanitized
+unterminated
 untrusted
 UTF
 uTidylib
diff --git a/docs/changelog.md b/docs/changelog.md
index a6939f91..c99a7c3c 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -17,6 +17,7 @@ See the [Contributing Guide](contributing.md) for details.
 * Update serializer to be non-recursive (#1644).
 * Improve ancestor handling in the inline `Treeprocessor` (#1646).
 * Fix issue where inline HTML attributes were rejected if they had `<` or `>` in the attribute (#1647).
+* Fix issue where an unterminated end tag (`</foo`) could cause all remaining content to be dropped (#1651).
 
 ## [3.11.0] - 2026-09-25
 
diff --git a/markdown/htmlparser.py b/markdown/htmlparser.py
index 488da033..d92dc0d6 100644
--- a/markdown/htmlparser.py
+++ b/markdown/htmlparser.py
@@ -99,10 +99,12 @@ class _HTMLParser(htmlparser.HTMLParser):
     def parse_endtag(self, i):
         start = self.rawdata[i:i+3]
         c = ord(start[-1])
-        if len(start) < 3 or not (65 <= c <= 90 or 97 <= c <= 122):
+        is_data = len(start) < 3 or not (65 <= c <= 90 or 97 <= c <= 122)
+        pos = super().parse_endtag(i) if not is_data else -1
+        if pos == -1:
             self.handle_data(self.rawdata[i:i + 2])
             return i + 2
-        return super().parse_endtag(i)
+        return pos
 
     def parse_starttag(self, i: int) -> int:  # pragma: no cover
         # Treat `</>` as normal data as it is not a real tag.
diff --git a/tests/test_syntax/blocks/test_html_blocks.py b/tests/test_syntax/blocks/test_html_blocks.py
index ecc10f45..1f2f8773 100644
--- a/tests/test_syntax/blocks/test_html_blocks.py
+++ b/tests/test_syntax/blocks/test_html_blocks.py
@@ -1787,3 +1787,60 @@ def test_unclosed_endtag(self):
                 '''
             )
         )
+
+    def test_unterminated_endtag_in_code_span(self):
+        """Ensure an unterminated end tag does not swallow the remaining content."""
+
+        # https://github.com/Python-Markdown/markdown/issues/1651
+        self.assertMarkdownRenders(
+            'Run `cmd 2</dev/null` here.\n\nThis paragraph should not disappear.',
+            self.dedent(
+                '''
+                <p>Run <code>cmd 2&lt;/dev/null</code> here.</p>
+                <p>This paragraph should not disappear.</p>
+                '''
+            )
+        )
+
+    def test_additional_tail_cases(self):
+        """Ensure various tail cases survive."""
+
+        # https://github.com/Python-Markdown/markdown/issues/1651
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                `</dev` z > q
+
+                `</ab c` z
+
+                `</dev/` > q
+
+                `</dev/null` z > q
+
+                `</ab c>` z
+                """
+            ),
+            self.dedent(
+                """
+                <p><code>&lt;/dev</code> z &gt; q</p>
+                <p><code>&lt;/ab c</code> z</p>
+                <p><code>&lt;/dev/</code> &gt; q</p>
+                <p><code>&lt;/dev/null</code> z &gt; q</p>
+                <p><code>&lt;/ab c&gt;</code> z</p>
+                """
+            )
+        )
+
+    def test_unterminated_endtag(self):
+        """Ensure an unterminated end tag is treated as text."""
+
+        # https://github.com/Python-Markdown/markdown/issues/1651
+        self.assertMarkdownRenders(
+            'foo </bar\n\nbaz',
+            self.dedent(
+                '''
+                <p>foo &lt;/bar</p>
+                <p>baz</p>
+                '''
+            )
+        )