diff --git a/cpp/src/arrow/compute/kernels/aggregate_basic_internal.h b/cpp/src/arrow/compute/kernels/aggregate_basic_internal.h
index 12d35b264e299..c1bbe9172f9dd 100644
--- a/cpp/src/arrow/compute/kernels/aggregate_basic_internal.h
+++ b/cpp/src/arrow/compute/kernels/aggregate_basic_internal.h
@@ -935,7 +935,7 @@ struct DictionaryMinMaxImpl : public ScalarAggregator {
     ARROW_ASSIGN_OR_RAISE(auto compacted_arr, dict_arr.Compact(ctx->memory_pool()));
     const DictionaryArray& compacted_dict_arr =
         checked_cast<const DictionaryArray&>(*compacted_arr);
-    this->has_nulls = compacted_dict_arr.null_count() > 0;
+    this->has_nulls |= compacted_dict_arr.null_count() > 0;
     this->count += compacted_dict_arr.length() - compacted_dict_arr.null_count();
 
     const std::shared_ptr<Array>& dict = compacted_dict_arr.dictionary();