diff --git a/modules/gltf/gltf_document.cpp b/modules/gltf/gltf_document.cpp
index e4c683f2645..1a9dacfa0b8 100644
--- a/modules/gltf/gltf_document.cpp
+++ b/modules/gltf/gltf_document.cpp
@@ -2833,13 +2833,17 @@ Error GLTFDocument::_parse_meshes(Ref<GLTFState> p_state) {
 			Vector<int> indices_vec4_mapping;
 			if (p.has("indices")) {
 				indices = _decode_accessor_as_ints(p_state, p["indices"], false);
-				const int is = indices.size();
+				const int index_count = indices.size();
 
 				if (primitive == Mesh::PRIMITIVE_TRIANGLES) {
+					if (index_count % 3 != 0) {
+						ERR_PRINT("glTF import: Mesh " + itos(i) + " surface " + itos(j) + " in file " + p_state->filename + " is invalid. Indexed triangle meshes MUST have an index array with a size that is a multiple of 3, but got " + itos(index_count) + " indices.");
+						continue;
+					}
 					// Swap around indices, convert ccw to cw for front face.
 
 					int *w = indices.ptrw();
-					for (int k = 0; k < is; k += 3) {
+					for (int k = 0; k < index_count; k += 3) {
 						SWAP(w[k + 1], w[k + 2]);
 					}
 				}
@@ -2848,7 +2852,7 @@ Error GLTFDocument::_parse_meshes(Ref<GLTFState> p_state) {
 				Vector<bool> used_indices;
 				used_indices.resize_zeroed(orig_vertex_num);
 				bool *used_w = used_indices.ptrw();
-				for (int idx_i = 0; idx_i < is; idx_i++) {
+				for (int idx_i = 0; idx_i < index_count; idx_i++) {
 					ERR_FAIL_INDEX_V(indices_w[idx_i], orig_vertex_num, ERR_INVALID_DATA);
 					used_w[indices_w[idx_i]] = true;
 				}
@@ -3048,11 +3052,15 @@ Error GLTFDocument::_parse_meshes(Ref<GLTFState> p_state) {
 				// Generate indices because they need to be swapped for CW/CCW.
 				const Vector<Vector3> &vertices = array[Mesh::ARRAY_VERTEX];
 				ERR_FAIL_COND_V(vertices.is_empty(), ERR_PARSE_ERROR);
-				const int vs = vertices.size();
-				indices.resize(vs);
+				const int vertex_count = vertices.size();
+				if (vertex_count % 3 != 0) {
+					ERR_PRINT("glTF import: Mesh " + itos(i) + " surface " + itos(j) + " in file " + p_state->filename + " is invalid. Non-indexed triangle meshes MUST have a vertex array with a size that is a multiple of 3, but got " + itos(vertex_count) + " vertices.");
+					continue;
+				}
+				indices.resize(vertex_count);
 				{
 					int *w = indices.ptrw();
-					for (int k = 0; k < vs; k += 3) {
+					for (int k = 0; k < vertex_count; k += 3) {
 						w[k] = k;
 						w[k + 1] = k + 2;
 						w[k + 2] = k + 1;
diff --git a/scene/resources/3d/importer_mesh.cpp b/scene/resources/3d/importer_mesh.cpp
index f912d2650da..6f7a597184d 100644
--- a/scene/resources/3d/importer_mesh.cpp
+++ b/scene/resources/3d/importer_mesh.cpp
@@ -307,6 +307,7 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, float p_normal_spli
 		if (index_count == 0) {
 			continue; //no lods if no indices
 		}
+		ERR_FAIL_COND_MSG(index_count % 3 != 0, "ImporterMesh::generate_lods: Indexed triangle meshes MUST have an index array with a size that is a multiple of 3, but got " + itos(index_count) + " indices. Cannot generate LODs for this invalid mesh.");
 
 		const Vector3 *vertices_ptr = vertices.ptr();
 		const int *indices_ptr = indices.ptr();