diff --git a/CHANGES b/CHANGES
index c75929ca6a3..4e5f7b29855 100644
--- a/CHANGES
+++ b/CHANGES
@@ -120,6 +120,8 @@ 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 60ede8188a4..9218e40c4d2 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 canon_path, os_path
 
 if TYPE_CHECKING:
     from sphinx.application import Sphinx
@@ -335,6 +336,7 @@ 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:
@@ -342,7 +344,7 @@ def relfn2path(self, filename: str, docname: str = None) -> Tuple[str, str]:
                                                 base=None))
             rel_fn = path.join(docdir, filename)
 
-        return (posixpath.normpath(rel_fn),
+        return (canon_path(path.normpath(rel_fn)),
                 path.normpath(path.join(self.srcdir, rel_fn)))
 
     @property