diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
index 8f8f9544677e..94253aefa3cc 100644
--- a/lib/matplotlib/pyplot.py
+++ b/lib/matplotlib/pyplot.py
@@ -1232,6 +1232,10 @@ def subplot(*args, **kwargs):
     # If no existing axes match, then create a new one.
     if ax is None:
         ax = fig.add_subplot(*args, **kwargs)
+    elif kwargs:
+        _api.warn_external(f"An Axes with subplot spec {args} already exists. "
+                           "The existing Axes will be returned, with all "
+                           "keyword arguments ignored.")
 
     bbox = ax.bbox
     axes_to_delete = []
diff --git a/lib/matplotlib/tests/test_pyplot.py b/lib/matplotlib/tests/test_pyplot.py
index 6b520aea30d4..1cfcb0d0035f 100644
--- a/lib/matplotlib/tests/test_pyplot.py
+++ b/lib/matplotlib/tests/test_pyplot.py
@@ -153,3 +153,11 @@ def test_nested_ion_ioff():
     with plt.ioff():
         plt.ion()
     assert not mpl.is_interactive()
+
+
+def test_subplot_warning():
+    ax1 = plt.subplot(2, 1, 1)
+    with pytest.warns(UserWarning, match='An Axes with subplot spec '
+                                         r'\(2, 1, 1\) already exists'):
+        ax2 = plt.subplot(2, 1, 1, polar=True)
+    assert ax1 is ax2