diff --git a/app/utils/template_filters.py b/app/utils/template_filters.py
index a71146fa..099ec136 100644
--- a/app/utils/template_filters.py
+++ b/app/utils/template_filters.py
@@ -25,7 +25,7 @@ def _normalize_toastui_markdown(text):
     """Clean up artefacts of Toast UI Editor's WYSIWYG-to-markdown conversion.
 
     Toast UI follows CommonMark and over-escapes punctuation when round-
-    tripping rich text through its editor. Two patterns matter for display:
+    tripping rich text through its editor. Three patterns matter for display:
 
     1. Line-leading ``\\- `` (also ``\\* ``, ``\\+ ``) prevents Python markdown
        from parsing the line as a list item. Toast UI emits this when the
@@ -34,6 +34,10 @@ def _normalize_toastui_markdown(text):
        (commas, colons, semicolons, etc.) renders as a literal ``\\,`` in
        the HTML output. Stripping the backslash is safe because none of
        those chars carry markdown meaning anywhere in a paragraph.
+    3. ``~~text~~`` strikethrough is part of CommonMark/GFM but the Python
+       ``markdown`` library's ``extra`` extension does not implement it,
+       so it leaks through as literal ``~~...~~`` text. Convert to inline
+       ``<del>`` HTML, which markdown passes through and bleach allows.
     """
     import re
 
@@ -43,6 +47,9 @@ def _normalize_toastui_markdown(text):
     text = re.sub(r"^(\s*)\\([\-+*])(\s)", r"\1\2\3", text, flags=re.MULTILINE)
     # Drop backslashes before punctuation Python markdown does not handle.
     text = re.sub(r"\\([^" + _PYMD_ESCAPABLE + r"])", r"\1", text)
+    # Convert ~~strikethrough~~ to <del>...</del>. Non-greedy and same-line
+    # only so multiple strikethroughs on one line each get their own pair.
+    text = re.sub(r"~~(.+?)~~", r"<del>\1</del>", text)
     return text