Cosmic Avenger is a side-scrolling shooter where you play as the pilot of a space fighter on a mission above a hostile alien planet. Your goal is to earn points by destroying enemies with your missiles and bombs. The aliens are armed with various deadly weapons, including missiles, UFOs, fuel domes, barrage guns, torpedoes, mines, and tanks.
The gameplay involves navigating the space fighter through the alien planet's atmosphere while avoiding the hostile enemies. You need to destroy as many of these enemies as possible before they destroy you. The game requires skill and strategy to maximize your score and survive for as long as possible.