Metal Gear Acid

Metal Gear Acid is the first Metal Gear to be released on the PlayStation Portable.

While the Metal Gear series, despite being stealth-based, has always had a heavy influence of action, Metal Gear Acid is not action-oriented, but focuses on turn-based strategy (this is possibly why the game's logo lacks the "Tactical Espionage Action" tagline featured on all of the Metal Gear Solid games). It uses a trading card-based system to control the main character's movements and actions.

Metal Gear Acid is a "side-story" and not part of the series' main storyline (similar to the earlier Metal Gear: Ghost Babel). While the game features an all-new cast, card sets are availble which features characters, weapons and items from each of the previously-released Metal Gear Solid titles, as well as a "Chronicle" set which covers Snatcher, Policenauts, Boktai, Zone of the Enders, Metal Gear: Ghost Babel and Metal Gear 1 and 2 are depicted on the cards used by the player.