diff --git a/sklearn/model_selection/_split.py b/sklearn/model_selection/_split.py
index bae48e5b93d77..b2e0f64af1c9d 100644
--- a/sklearn/model_selection/_split.py
+++ b/sklearn/model_selection/_split.py
@@ -1183,7 +1183,9 @@ class TimeSeriesSplit(_BaseKFold):
     The training set has size ``i * n_samples // (n_splits + 1)
     + n_samples % (n_splits + 1)`` in the ``i`` th split,
     with a test set of size ``n_samples//(n_splits + 1)`` by default,
-    where ``n_samples`` is the number of samples.
+    where ``n_samples`` is the number of samples. Note that this
+    formula is only valid when ``test_size`` and ``max_train_size`` are
+    left to their default values.
     """
 
     def __init__(self, n_splits=5, *, max_train_size=None, test_size=None, gap=0):