diff --git a/src/_pytest/fixtures.py b/src/_pytest/fixtures.py
index d8d19fcac6d..84f90f946be 100644
--- a/src/_pytest/fixtures.py
+++ b/src/_pytest/fixtures.py
@@ -1635,8 +1635,8 @@ def pytest_plugin_registered(self, plugin: _PluggyPlugin, plugin_name: str) -> N
                 nodeid = ""
             if nodeid == ".":
                 nodeid = ""
-            if os.sep != nodes.SEP:
-                nodeid = nodeid.replace(os.sep, nodes.SEP)
+            elif nodeid:
+                nodeid = nodes.norm_sep(nodeid)
         else:
             nodeid = None
 
diff --git a/src/_pytest/nodes.py b/src/_pytest/nodes.py
index 6690f6ab1f8..bc1dfc90d96 100644
--- a/src/_pytest/nodes.py
+++ b/src/_pytest/nodes.py
@@ -51,6 +51,20 @@
 
 SEP = "/"
 
+
+def norm_sep(path: str | os.PathLike[str]) -> str:
+    """Normalize path separators to forward slashes for nodeid compatibility.
+
+    Replaces backslashes with forward slashes. This handles both Windows native
+    paths and cross-platform data (e.g., Windows paths in serialized test reports
+    when running on Linux).
+
+    :param path: A path string or PathLike object.
+    :returns: String with all backslashes replaced by forward slashes.
+    """
+    return os.fspath(path).replace("\\", SEP)
+
+
 tracebackcutdir = Path(_pytest.__file__).parent
 
 
@@ -589,7 +603,7 @@ def __init__(
                     pass
                 else:
                     name = str(rel)
-                name = name.replace(os.sep, SEP)
+                name = norm_sep(name)
         self.path = path
 
         if session is None:
@@ -602,8 +616,8 @@ def __init__(
             except ValueError:
                 nodeid = _check_initialpaths_for_relpath(session._initialpaths, path)
 
-            if nodeid and os.sep != SEP:
-                nodeid = nodeid.replace(os.sep, SEP)
+            if nodeid:
+                nodeid = norm_sep(nodeid)
 
         super().__init__(
             name=name,
diff --git a/src/_pytest/terminal.py b/src/_pytest/terminal.py
index 837a78cc568..bb6f35633b9 100644
--- a/src/_pytest/terminal.py
+++ b/src/_pytest/terminal.py
@@ -1034,9 +1034,7 @@ def mkrel(nodeid: str) -> str:
         # fspath comes from testid which has a "/"-normalized path.
         if fspath:
             res = mkrel(nodeid)
-            if self.verbosity >= 2 and nodeid.split("::")[0] != fspath.replace(
-                "\\", nodes.SEP
-            ):
+            if self.verbosity >= 2 and nodeid.split("::")[0] != nodes.norm_sep(fspath):
                 res += " <- " + bestrelpath(self.startpath, Path(fspath))
         else:
             res = "[location]"
diff --git a/testing/test_nodes.py b/testing/test_nodes.py
index de7875ca427..f66a11ce5c8 100644
--- a/testing/test_nodes.py
+++ b/testing/test_nodes.py
@@ -98,6 +98,37 @@ def test():
         items[0].warn(Exception("ok"))  # type: ignore[arg-type]
 
 
+class TestNormSep:
+    """Tests for the norm_sep helper function."""
+
+    def test_forward_slashes_unchanged(self) -> None:
+        """Forward slashes pass through unchanged."""
+        assert nodes.norm_sep("a/b/c") == "a/b/c"
+
+    def test_backslashes_converted(self) -> None:
+        """Backslashes are converted to forward slashes."""
+        assert nodes.norm_sep("a\\b\\c") == "a/b/c"
+
+    def test_mixed_separators(self) -> None:
+        """Mixed separators are all normalized to forward slashes."""
+        assert nodes.norm_sep("a\\b/c\\d") == "a/b/c/d"
+
+    def test_pathlike_input(self, tmp_path: Path) -> None:
+        """PathLike objects are converted to string with normalized separators."""
+        # Create a path and verify it's normalized
+        result = nodes.norm_sep(tmp_path / "subdir" / "file.py")
+        assert "\\" not in result
+        assert "subdir/file.py" in result
+
+    def test_empty_string(self) -> None:
+        """Empty string returns empty string."""
+        assert nodes.norm_sep("") == ""
+
+    def test_windows_absolute_path(self) -> None:
+        """Windows absolute paths have backslashes converted."""
+        assert nodes.norm_sep("C:\\Users\\test\\project") == "C:/Users/test/project"
+
+
 def test__check_initialpaths_for_relpath() -> None:
     """Ensure that it handles dirs, and does not always use dirname."""
     cwd = Path.cwd()