diff --git a/CHANGES.md b/CHANGES.md
index b1413c29a5..fbc16da121 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -26,6 +26,7 @@ Core Grammars:
 - enh(json) add json5 support [Kerry Shetline][]
 - fix(css) `unicode-range` parsing, issue #4253 [Kerry Shetline][]
 - fix(csharp) Support digit separators [te-ing][]
+- fix(markdown) Ignore mid_word underscores, issue #4279 [Dan Vanderkam]
 
 Documentation:
 
@@ -55,6 +56,7 @@ CONTRIBUTORS
 [te-ing]: https://github.com/te-ing
 [Anthony Martin]: https://github.com/anthony-c-martin
 [NriotHrreion]: https://github.com/NriotHrreion
+[Dan Vanderkam]: https://github.com/danvk
 
 
 ## Version 11.11.1
diff --git a/src/languages/markdown.js b/src/languages/markdown.js
index 3d73d062e9..5a08a3ff72 100644
--- a/src/languages/markdown.js
+++ b/src/languages/markdown.js
@@ -156,8 +156,8 @@ export default function(hljs) {
         end: /\*/
       },
       {
-        begin: /_(?![_\s])/,
-        end: /_/,
+        begin: /(?<![a-zA-Z0-9])_(?![_\s])/,
+        end: /_(?![a-zA-Z0-9])/,
         relevance: 0
       }
     ]
diff --git a/test/markup/markdown/bold_italics.expect.txt b/test/markup/markdown/bold_italics.expect.txt
index a08ae07756..37461d4df2 100644
--- a/test/markup/markdown/bold_italics.expect.txt
+++ b/test/markup/markdown/bold_italics.expect.txt
@@ -31,3 +31,6 @@ _ not italic_
 <span class="hljs-quote">&gt; * One (this point is italic)</span>
 <span class="hljs-quote">&gt; * Two</span>
 <span class="hljs-quote">&gt; * Three</span>
+
+No italics mid_word underscores.
+But italics for <span class="hljs-emphasis">_full_word_</span> with underscores.
diff --git a/test/markup/markdown/bold_italics.txt b/test/markup/markdown/bold_italics.txt
index c85202e554..fa44e30c83 100644
--- a/test/markup/markdown/bold_italics.txt
+++ b/test/markup/markdown/bold_italics.txt
@@ -31,3 +31,6 @@ _ not italic_
 > * One (this point is italic)
 > * Two
 > * Three
+
+No italics mid_word underscores.
+But italics for _full_word_ with underscores.