diff --git a/stubs/rasterio/@tests/test_cases/check_crs_input.py b/stubs/rasterio/@tests/test_cases/check_crs_input.py
new file mode 100644
index 000000000000..474c927bf952
--- /dev/null
+++ b/stubs/rasterio/@tests/test_cases/check_crs_input.py
@@ -0,0 +1,23 @@
+from __future__ import annotations
+
+from typing_extensions import assert_type
+
+import rasterio.warp
+from rasterio.crs import CRS
+from rasterio.io import MemoryFile
+
+
+class ForeignCRS:
+    def to_wkt(self) -> str:
+        return ""
+
+
+assert_type(rasterio.warp.transform_bounds(4326, 3857, 0.0, 0.0, 1.0, 1.0), tuple[float, float, float, float])
+rasterio.warp.transform_bounds(ForeignCRS(), ForeignCRS(), 0.0, 0.0, 1.0, 1.0)
+rasterio.warp.transform_bounds(CRS.from_epsg(4326), "EPSG:3857", 0.0, 0.0, 1.0, 1.0)
+rasterio.warp.transform_bounds({"init": "EPSG:4326"}, CRS.from_epsg(3857), 0.0, 0.0, 1.0, 1.0)
+rasterio.warp.transform_bounds(object(), 3857, 0.0, 0.0, 1.0, 1.0)  # type: ignore
+rasterio.warp.transform(4326, ForeignCRS(), [0.0], [0.0])
+rasterio.warp.calculate_default_transform(4326, ForeignCRS(), 10, 10, left=0.0, bottom=0.0, right=1.0, top=1.0)
+MemoryFile().open(driver="GTiff", width=1, height=1, count=1, dtype="uint8", crs=4326)
+MemoryFile().open(driver="GTiff", width=1, height=1, count=1, dtype="uint8", crs=ForeignCRS())
diff --git a/stubs/rasterio/rasterio/_typing.pyi b/stubs/rasterio/rasterio/_typing.pyi
index 974afc3a8369..a1011123b397 100644
--- a/stubs/rasterio/rasterio/_typing.pyi
+++ b/stubs/rasterio/rasterio/_typing.pyi
@@ -16,13 +16,19 @@ class _SupportsGeoInterface(Protocol):
     @property
     def __geo_interface__(self) -> Mapping[str, Any]: ...
 
+@type_check_only
+class _SupportsToWkt(Protocol):
+    def to_wkt(self) -> str: ...
+
 # A GeoJSON-like mapping, or any object exposing one through the
 # `__geo_interface__` protocol (e.g. shapely / geopandas geometries).
 # The runtime unwraps `__geo_interface__` before use, so both forms are
 # accepted anywhere a geometry is expected.
 Geometry: TypeAlias = Mapping[str, Any] | _SupportsGeoInterface  # noqa: Y047
 Colormap: TypeAlias = dict[int, tuple[int, int, int] | tuple[int, int, int, int]]
-CRSInput: TypeAlias = str | dict[str, str] | CRS
+# A WKT / PROJ string, an EPSG code, a PROJ dict, or any object exposing
+# `to_wkt` (e.g. a pyproj CRS). `CRS.from_user_input` normalizes all of them.
+CRSInput: TypeAlias = str | int | dict[str, str] | CRS | _SupportsToWkt
 FileOrBytes: TypeAlias = BinaryIO | bytes
 Indexes: TypeAlias = int | Sequence[int]
 NumType: TypeAlias = int | float