Xenosaga I + II combines the storylines of the first and second PlayStation 2 episodes in one game.

The battle system is similar to the one found in Episode I, with a change that your AP meter now goes up to a maximum of 6 levels. New characters and new skills have been implemented. Instead of combination attacks as present in the PS2 versions, you can now perform Personal Formations, in which the formation of your battle party affects your surroundings.

The game features animated movies and voices as cut scenes. Stylus supports has been added to play dungeon mini games.

The script for Xenosaga I + II has been written by Yuichiro Takeda, who also worked on the Xenosaga anime. Character designs (which are different from the two PS2 titles) are by Hiroshi Takeuchi. Kousuke Yamashita, of Nobunaga's Ambition fame has been responsible for the music.