diff --git a/altair/utils/schemapi.py b/altair/utils/schemapi.py
index 8b1c2ebb3..2b243ed92 100644
--- a/altair/utils/schemapi.py
+++ b/altair/utils/schemapi.py
@@ -982,7 +982,7 @@ def to_dict(
     def to_json(
         self,
         validate: bool = True,
-        indent: int = 2,
+        indent: Optional[Union[int, str]] = 2,
         sort_keys: bool = True,
         *,
         ignore: Optional[List[str]] = None,
diff --git a/altair/vegalite/v5/api.py b/altair/vegalite/v5/api.py
index 05e088b6d..13be9421f 100644
--- a/altair/vegalite/v5/api.py
+++ b/altair/vegalite/v5/api.py
@@ -1003,7 +1003,7 @@ def to_dict(
     def to_json(
         self,
         validate: bool = True,
-        indent: int = 2,
+        indent: Optional[Union[int, str]] = 2,
         sort_keys: bool = True,
         *,
         format: str = "vega-lite",
diff --git a/tools/schemapi/schemapi.py b/tools/schemapi/schemapi.py
index 67365c762..e3a802594 100644
--- a/tools/schemapi/schemapi.py
+++ b/tools/schemapi/schemapi.py
@@ -980,7 +980,7 @@ def to_dict(
     def to_json(
         self,
         validate: bool = True,
-        indent: int = 2,
+        indent: Optional[Union[int, str]] = 2,
         sort_keys: bool = True,
         *,
         ignore: Optional[List[str]] = None,