Preheat an outdoor grill to medium heat. Cut two sheets of heavy duty aluminum foil, each approximately 18 inches long.
Place the yellow squash, zucchini, tomatoes, onion, and garlic in the center of one sheet of aluminum foil. Drizzle with olive oil; season with salt and pepper. Place second sheet of foil on top of vegetables. Fold edges together tightly to seal packet.
Place packet on the top rack of the preheated grill. Cook for 15 minutes. Turn packet over and continue to cook until vegetables are tender, 15 to 20 minutes.
Carefully cut an "X" into top of packet and pour vegetables and juices into serving bowl. Garnish with parsley.
