diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 6acb231bcd5..9e7b1de889f 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -108,6 +108,11 @@ Breaking changes
 Deprecations
 ~~~~~~~~~~~~
 
+- The `lock` keyword argument to :py:func:`open_dataset` and :py:func:`open_dataarray` has now
+  been deprecated, and will give a warning if passed. From the next version it will
+  raise an error. This is part of the refactor to support external backends (:issue:`5073`).
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
+
 Bug fixes
 ~~~~~~~~~
 - Properly support :py:meth:`DataArray.ffill`, :py:meth:`DataArray.bfill`, :py:meth:`Dataset.ffill`, :py:meth:`Dataset.bfill` along chunked dimensions.
diff --git a/xarray/backends/api.py b/xarray/backends/api.py
index 01079025434..b858bfabdb4 100644
--- a/xarray/backends/api.py
+++ b/xarray/backends/api.py
@@ -1,4 +1,5 @@
 import os
+import warnings
 from glob import glob
 from io import BytesIO
 from numbers import Number
@@ -444,11 +445,6 @@ def open_dataset(
 
         - 'group': path to the netCDF4 group in the given file to open given as
           a str,supported by "netcdf4", "h5netcdf", "zarr".
-        - 'lock': resource lock to use when reading data from disk. Only
-          relevant when using dask or another form of parallelism. By default,
-          appropriate locks are chosen to safely read and write files with the
-          currently active dask scheduler. Supported by "netcdf4", "h5netcdf",
-          "pynio", "pseudonetcdf", "cfgrib".
 
         See engine open function for kwargs accepted by each specific engine.
 
@@ -474,6 +470,15 @@ def open_dataset(
             "all other options must be passed as keyword arguments"
         )
 
+    # TODO remove after v0.19
+    if kwargs.pop("lock", None):
+        warnings.warn(
+            "The kwarg 'lock' has been deprecated, and is now "
+            "ignored. In the future passing lock will "
+            "raise an error.",
+            DeprecationWarning,
+        )
+
     if cache is None:
         cache = chunks is None
 
@@ -628,11 +633,6 @@ def open_dataarray(
 
         - 'group': path to the netCDF4 group in the given file to open given as
           a str,supported by "netcdf4", "h5netcdf", "zarr".
-        - 'lock': resource lock to use when reading data from disk. Only
-          relevant when using dask or another form of parallelism. By default,
-          appropriate locks are chosen to safely read and write files with the
-          currently active dask scheduler. Supported by "netcdf4", "h5netcdf",
-          "pynio", "pseudonetcdf", "cfgrib".
 
         See engine open function for kwargs accepted by each specific engine.
 
@@ -655,6 +655,15 @@ def open_dataarray(
             "all other options must be passed as keyword arguments"
         )
 
+        # TODO remove after v0.19
+        if kwargs.pop("lock", None):
+            warnings.warn(
+                "The kwarg 'lock' has been deprecated, and is now "
+                "ignored. In the future passing lock will "
+                "raise an error.",
+                DeprecationWarning,
+            )
+
     dataset = open_dataset(
         filename_or_obj,
         decode_cf=decode_cf,