diff --git a/py-polars/polars/dataframe/frame.py b/py-polars/polars/dataframe/frame.py
index 9ca0ae0e7e65..4034f0331d0e 100644
--- a/py-polars/polars/dataframe/frame.py
+++ b/py-polars/polars/dataframe/frame.py
@@ -11535,8 +11535,9 @@ def merge_sorted(self, other: DataFrame, key: str) -> DataFrame:
         Take two sorted DataFrames and merge them by the sorted key.
 
         The output of this operation will also be sorted.
-        It is the callers responsibility that the frames are sorted
-        by that key otherwise the output will not make sense.
+        It is the callers responsibility that the frames
+        are sorted in ascending order by that key otherwise
+        the output will not make sense.
 
         The schemas of both DataFrames must be equal.
 
@@ -11598,6 +11599,8 @@ def merge_sorted(self, other: DataFrame, key: str) -> DataFrame:
         -----
         No guarantee is given over the output row order when the key is equal
         between the both dataframes.
+
+        The key must be sorted in ascending order.
         """
         return self.lazy().merge_sorted(other.lazy(), key).collect(_eager=True)
 
diff --git a/py-polars/polars/lazyframe/frame.py b/py-polars/polars/lazyframe/frame.py
index 46ee33f618c1..cc4df14fdce6 100644
--- a/py-polars/polars/lazyframe/frame.py
+++ b/py-polars/polars/lazyframe/frame.py
@@ -7096,8 +7096,9 @@ def merge_sorted(self, other: LazyFrame, key: str) -> LazyFrame:
         Take two sorted DataFrames and merge them by the sorted key.
 
         The output of this operation will also be sorted.
-        It is the callers responsibility that the frames are sorted
-        by that key otherwise the output will not make sense.
+        It is the callers responsibility that the frames
+        are sorted in ascending order by that key otherwise
+        the output will not make sense.
 
         The schemas of both LazyFrames must be equal.
 
@@ -7159,6 +7160,8 @@ def merge_sorted(self, other: LazyFrame, key: str) -> LazyFrame:
         -----
         No guarantee is given over the output row order when the key is equal
         between the both dataframes.
+
+        The key must be sorted in ascending order.
         """
         return self._from_pyldf(self._ldf.merge_sorted(other._ldf, key))