Gradius Galaxies is a scrolling shooter game developed by Mobile21 and published by Konami in 2001. It is the only Gradius title available for the Game Boy Advance. The game's plot is set between Gradius III and Gradius Gaiden. Bacterion was developing a powerful weapon to use against the planet Gradius, but it was destroyed. A few years later it crashed on a planet and the planet gradually changed into a mechanical fortress. The planet Gradius then sent the Vic Viper to stop it.
The gameplay is almost identical to other titles in the series. The player takes control of the Vic Viper spacecraft tasked with destroying hordes of enemies. Various traditional elements of Gradius are present — the moai, an undefended final boss, and the upgrade power meter. The weapons system remained mostly unchanged as the configurations available are identical to Gradius III, with the exception that the edit mode is missing. There are four different types of weapons configurations: Balanced, (traditional Gradius) wide area (Salamander) power, and air-to-ground. Each configuration is indicated by the color of Vic Viper. Unlike Gradius III, there are only two shield options — shield and force field. In addition, the S. option ("snake option") is not present in this version; the player can however choose to have the computer assign weapon power-ups automatically or purchase upgrades manually.
The Japanese version included a challenge game play mode that had to be unlocked by achieving a high score during a single play-through of the game's normal difficulty mode. Additional challenges could then by unlocked by completing other challenges. This feature was absent from the U.S. and European releases due to the game being released in Japan several months later.