diff --git a/cpp/src/arrow/array/array_list_test.cc b/cpp/src/arrow/array/array_list_test.cc
index 8406bd1d8ed1..9908eb588655 100644
--- a/cpp/src/arrow/array/array_list_test.cc
+++ b/cpp/src/arrow/array/array_list_test.cc
@@ -1532,6 +1532,43 @@ TEST_F(TestMapArray, ValueBuilder) {
   ASSERT_ARRAYS_EQUAL(*actual_list, map_as_list);
 }
 
+// GH-51029: Validate MapArray keys with an all-valid bitmap and unknown null count.
+TEST_F(TestMapArray, ValidateKeysWithAllValidBitmap) {
+  auto keys = ArrayFromJSON(utf8(), R"(["a", "b"])");
+  auto items = ArrayFromJSON(int32(), "[1, 2]");
+  auto offsets = ArrayFromJSON(int32(), "[0, 1, 2]");
+
+  // Inject an all-valid validity bitmap with kUnknownNullCount.
+  auto keys_data = keys->data()->Copy();
+  keys_data->buffers[0] = ArrayFromJSON(boolean(), "[true, true]")->data()->buffers[1];
+  keys_data->null_count = kUnknownNullCount;
+
+  ASSERT_OK_AND_ASSIGN(auto result,
+                       MapArray::FromArrays(offsets, MakeArray(keys_data), items));
+  ASSERT_OK(result->ValidateFull());
+  ASSERT_EQ(result->length(), 2);
+}
+
+TEST_F(TestMapArray, FromArraysWithAllValidOffsetsBitmap) {
+  auto offsets = ArrayFromJSON(int32(), "[0, 1, 2]");
+  auto keys = ArrayFromJSON(utf8(), R"(["a", "b"])");
+  auto items = ArrayFromJSON(int32(), "[1, 2]");
+
+  // Inject an all-valid validity bitmap with kUnknownNullCount.
+  auto offsets_data = offsets->data()->Copy();
+  offsets_data->buffers[0] =
+      ArrayFromJSON(boolean(), "[true, true, true]")->data()->buffers[1];
+  offsets_data->null_count = kUnknownNullCount;
+  offsets = MakeArray(offsets_data);
+
+  auto null_bitmap = ArrayFromJSON(boolean(), "[true, true]")->data()->buffers[1];
+
+  ASSERT_OK_AND_ASSIGN(auto result,
+                       MapArray::FromArrays(offsets, keys, items, pool_, null_bitmap));
+  ASSERT_OK(result->ValidateFull());
+  ASSERT_EQ(result->length(), 2);
+}
+
 // ----------------------------------------------------------------------
 // FixedSizeList tests
 
diff --git a/cpp/src/arrow/array/array_nested.cc b/cpp/src/arrow/array/array_nested.cc
index c5a26a475c9e..55bce4d0def7 100644
--- a/cpp/src/arrow/array/array_nested.cc
+++ b/cpp/src/arrow/array/array_nested.cc
@@ -115,7 +115,7 @@ Result<std::shared_ptr<typename TypeTraits<TYPE>::ArrayType>> ListArrayFromArray
     return Status::TypeError("List offsets must be ", OffsetArrowType::type_name());
   }
 
-  if (null_bitmap != nullptr && offsets.data()->MayHaveNulls()) {
+  if (null_bitmap != nullptr && offsets.data()->GetNullCount() != 0) {
     return Status::Invalid(
         "Ambiguous to specify both validity map and offsets with nulls");
   }
@@ -826,7 +826,7 @@ Result<std::shared_ptr<Array>> MapArray::FromArraysInternal(
     return Status::Invalid("Map key and item arrays must be equal length");
   }
 
-  if (null_bitmap != nullptr && offsets->data()->MayHaveNulls()) {
+  if (null_bitmap != nullptr && offsets->data()->GetNullCount() != 0) {
     return Status::Invalid(
         "Ambiguous to specify both validity map and offsets with nulls");
   }
@@ -835,7 +835,7 @@ Result<std::shared_ptr<Array>> MapArray::FromArraysInternal(
     return Status::NotImplemented("Null bitmap with offsets slice not supported.");
   }
 
-  if (offsets->data()->MayHaveNulls()) {
+  if (offsets->data()->GetNullCount() != 0) {
     ARROW_ASSIGN_OR_RAISE(auto buffers,
                           CleanListOffsets<MapType>(NULLPTR, *offsets, pool));
     return std::make_shared<MapArray>(type, offsets->length() - 1, std::move(buffers),
@@ -896,13 +896,13 @@ Status MapArray::ValidateChildData(
   if (pair_data->type->id() != Type::STRUCT) {
     return Status::Invalid("Map array child array should have struct type");
   }
-  if (pair_data->MayHaveNulls()) {
+  if (pair_data->GetNullCount() != 0) {
     return Status::Invalid("Map array child array should have no nulls");
   }
   if (pair_data->child_data.size() != 2) {
     return Status::Invalid("Map array child array should have two fields");
   }
-  if (pair_data->child_data[0]->MayHaveNulls()) {
+  if (pair_data->child_data[0]->GetNullCount() != 0) {
     return Status::Invalid("Map array keys array should have no nulls");
   }
   return Status::OK();