diff --git a/_includes/seo.html b/_includes/seo.html
index a43115b9..5568807e 100644
--- a/_includes/seo.html
+++ b/_includes/seo.html
@@ -39,7 +39,7 @@
   {% assign seo_author_twitter = seo_author_twitter | replace: "@", "" %}
 {% endif %}
 
-<meta property="og:locale" content="{{ site.locale | default: "en" }}">
+<meta property="og:locale" content="{{ site.locale | replace: "-", "_" | default: "en" }}">
 <meta property="og:site_name" content="{{ site.title }}">
 <meta property="og:title" content="{{ page.title | default: site.title | markdownify | strip_html | strip_newlines | escape_once }}">