If you buy frozen meatballs, most are already cooked. However, it still is a good idea to heat frozen meatballs until the internal temperature is 165 degrees. To do this, preheat the oven to 350. Place the number of frozen meatballs you want in a single layer on a parchment-lined baking sheet. Be sure not to have the meatballs too close together; there should be an inch or more space between them—Bake at 350 degrees for 20 minutes. Remove from the baking sheet and toss them into your sauce.