diff --git a/CHANGELOG.md b/CHANGELOG.md
index 243339032..0f8efa51d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- `DataCube.apply_dimension()`: explicitly specifying the `dimension` argument is now mandatory. The old default (dimension "t") was unnecessarily opinionated and causing confusion about how the process handles dimensions. ([#774](https://github.com/Open-EO/openeo-python-client/issues/774))
+
 ### Removed
 
 ### Fixed
diff --git a/openeo/rest/datacube.py b/openeo/rest/datacube.py
index 4be7b3469..791a62bea 100644
--- a/openeo/rest/datacube.py
+++ b/openeo/rest/datacube.py
@@ -1366,14 +1366,14 @@ def aggregate_spatial_window(
     def apply_dimension(
         self,
         code: Optional[str] = None,
-        runtime=None,
+        *,
         # TODO: drop None default of process (when `code` and `runtime` args can be dropped)
         process: Union[str, typing.Callable, UDF, PGNode] = None,
-        version: Optional[str] = None,
-        # TODO: dimension has no default (per spec)?
-        dimension: str = "t",
+        dimension: str,
         target_dimension: Optional[str] = None,
         context: Optional[dict] = None,
+        runtime: Optional[str] = None,
+        version: Optional[str] = None,
     ) -> DataCube:
         """
         Applies a process to all pixel values along a dimension of a raster data cube. For example,
@@ -1426,6 +1426,21 @@ def apply_dimension(
             of using an :py:class:`UDF <openeo.rest._datacube.UDF>` object in the ``process`` argument.
             See :ref:`old_udf_api` for more background about the changes.
 
+        .. versionchanged:: 0.42.0
+            Argument ``dimension`` is mandatory now.
+            It used to have default value "t" but that was overly opinionated
+            and causing confusion about how dimensions are handled.
+
+            As related side-effect, and as further deprecation of the old ``code``/``runtime``/``version`` API,
+            at most one positional argument is allowed now.
+            This argument still has its original name ``code``,
+            but it's likely it will be renamed to ``process`` in the future.
+            For optimal future (and backward) compatibility,
+            it is recommended to always use keyword arguments,
+            e.g. with at least ``process`` and ``dimension``:
+
+            .. code-block:: python
+                cube.apply_dimension(process=..., dimension=...)
         """
         # TODO #137 #181 #312 remove support for code/runtime/version
         if runtime or (isinstance(code, str) and "\n" in code) or version: