Mix beef, rice, onion, egg, garlic, salt, and pepper in a bowl until combined. Divide mixture into 8 equal portions and form into meatballs.
Place meatballs in a multi-functional pressure cooker (such as Instant Pot®). Mix tomato soup and water and pour over meatballs.
Close and lock the lid. Select high pressure according to manufacturer’s instructions; set timer for 10 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer’s instructions, 10 to 40 minutes.