Galaxian expanded on the formula pioneered by Space Invaders. As in the earlier game Galaxian features a horde of attacking aliens that exchanged shots with the player. In contrast to Space Invaders Galaxian added an element of drama by having the aliens periodically make kamikaze-like dives at the player's ship the Galaxip. This made it the first game to feature enemies with individual personalities. The game's plot consists of a title screen that displayed the message "We are the Galaxians / Mission:Destroy Aliens".

Swarm after swarm of alien armies attack the player's ship that moves left and right at the bottom of the wraparound screen. The Galaxip can only have one shot on screen at a time. The player defeats one swarm only to have it replaced by another more aggressive and challenging swarm in the next stage. A plain and repetitive starfield scrolls in the background.