diff --git a/CHANGES b/CHANGES
index 8fa86a98b26..fe9e8e28bca 100644
--- a/CHANGES
+++ b/CHANGES
@@ -13,6 +13,8 @@ Deprecated
 Features added
 --------------
 
+* #8959: using UNIX path separator in image directive confuses Sphinx on Windows
+
 Bugs fixed
 ----------
 
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index af3e2b8d542..6058b541fd2 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -34,6 +34,7 @@
 from sphinx.util.docutils import LoggingReporter
 from sphinx.util.i18n import CatalogRepository, docname_to_domain
 from sphinx.util.nodes import is_translatable
+from sphinx.util.osutil import os_path
 
 if False:
     # For type annotation
@@ -351,6 +352,7 @@ def relfn2path(self, filename: str, docname: str = None) -> Tuple[str, str]:
         source dir, while relative filenames are relative to the dir of the
         containing document.
         """
+        filename = os_path(filename)
         if filename.startswith('/') or filename.startswith(os.sep):
             rel_fn = filename[1:]
         else: