Metal Slug XX (reads Metal Slug Double X) is a enhanced modification of Metal Slug 7, released in December 2009 for the Playstation Portable and Xbox Live Arcade with new content.
- The screen is not re-dimensioned anymore, like in the Nintendo DS. Additionally, the player can select the aspect ratio of the screen:
- 4:3 (Classic aspect ratio with designed borders at left and right of the screen)
- Wide (Fullscreen)
- Clear (Like "4:3" but with a smaller screen for better visual quality)
- Rebel Troops reuse their sound effects from Metal Slug 6 (PSP only).
- Old enemies reappear again, such as the Mars People and the Hunters.
- Change in items, slugs, and enemy placement.
- Multiplayer mode by using ad hoc and online services.
- Leona Heidern as a downloadable character (Costs $0.99).
- New missions in the Combat School mode.
- New Hidden Routes:
- Mission 1 - There should be a pitfall/hole before the Di-Cokka and the DANGER barrel. Go there and fight the hunters.
- Mission 5a - When you reach the end of the second part of the stage, it says GO right. Instead, go left until you find a teleporter. Go in the teleporter and fight UFOs and tons of soldiers.
- Mission 5b - Just like Mission 5a but do not go in the teleporter. Jump over it. You should fight hordes of Mars Peoples in there.
- Metal Slug XX is the third installment in the series to increase the number of playable characters, from 6 to 7 with the addition of Leona.