Galaxian is a shoot 'em up where you're at the bottom of the screen, facing an alien arrangement at the top. Your goal is to move left and right, aim at an alien, and fire a bullet up the screen to hit it. Different aliens have different point values when destroyed, with the higher ones worth more points. Sometimes, enemies will move down the screen towards you, accompanied by a bomber that's a high-risk situation.
Gameplay involves clearing waves of aliens, with each wave being followed by another one as soon as the previous one is defeated. You'll need to stay focused and quick on the draw to take down the aliens and avoid the bombers.