diff --git a/modin/core/dataframe/pandas/partitioning/partition_manager.py b/modin/core/dataframe/pandas/partitioning/partition_manager.py
index a642f9453f1..904a2809b4f 100644
--- a/modin/core/dataframe/pandas/partitioning/partition_manager.py
+++ b/modin/core/dataframe/pandas/partitioning/partition_manager.py
@@ -257,8 +257,16 @@ def groupby_reduce(
             )
         else:
             mapped_partitions = cls.map_partitions(partitions, map_func)
+
+        # Assuming, that the output will not be larger than the input,
+        # keep the current number of partitions.
+        num_splits = min(len(partitions), NPartitions.get())
         return cls.map_axis_partitions(
-            axis, mapped_partitions, reduce_func, enumerate_partitions=True
+            axis,
+            mapped_partitions,
+            reduce_func,
+            enumerate_partitions=True,
+            num_splits=num_splits,
         )
 
     @classmethod