diff --git a/changelogs/fragments/11995-cobbler-system-interface-keyerror.yml b/changelogs/fragments/11995-cobbler-system-interface-keyerror.yml
new file mode 100644
index 0000000000..0e93530a98
--- /dev/null
+++ b/changelogs/fragments/11995-cobbler-system-interface-keyerror.yml
@@ -0,0 +1,4 @@
+bugfixes:
+  - cobbler_system - fix ``KeyError`` when adding a new interface to an existing system that does not yet have it defined
+    (https://github.com/ansible-collections/community.general/issues/7007,
+    https://github.com/ansible-collections/community.general/pull/11995).
diff --git a/plugins/modules/cobbler_system.py b/plugins/modules/cobbler_system.py
index 25eb8aa72d..8070755281 100644
--- a/plugins/modules/cobbler_system.py
+++ b/plugins/modules/cobbler_system.py
@@ -298,7 +298,8 @@ def main():
                         continue
                     if key not in IFPROPS_MAPPING:
                         module.warn(f"Property '{key}' is not a valid system property.")
-                    if not system or system["interfaces"][device][IFPROPS_MAPPING[key]] != value:
+                        continue
+                    if not system or system["interfaces"].get(device, {}).get(IFPROPS_MAPPING[key]) != value:
                         result["changed"] = True
                     interface_properties[f"{key}-{device}"] = value