Gather all ingredients.
Place ground beef, bread crumbs, onion, garlic, parsley, and egg in a large bowl; mix well until evenly combined, then shape into 16 meatballs.
Pour spaghetti sauce and crushed tomatoes into the slow cooker; stir in tomato purée. Place the meatballs into the sauce mixture.
Cover and cook on Low for 6 to 8 hours.
