If using frozen fully cooked meatballs, there’s no need to thaw them. Simply place them directly into the slow cooker. If you’re using homemade meatballs, make sure they’re fully cooked before adding them in.
Pour the ketchup and grape jelly over the meatballs in the slow cooker. Stir gently until the meatballs are evenly coated and the sauce begins to combine.
Cover the slow cooker and cook on low for 3 to 4 hours. Stir occasionally to help the sauce blend smoothly and ensure every meatball is coated in that glossy, flavorful glaze.