Metal Gear Solid: Portable Ops (メタルギアソリッド ポータブルオプス, Metaru Gia Soriddo Pōtaburu Opusu) (commonly abbreviated to MPO) is a stealth game directed by Masahiro Yamamoto, with series creator Hideo Kojima acting as a producer. Portable Ops was developed by Kojima Productions and published by Konami in 2006 for the PlayStation Portable. It is the third Metal Gear title for the PSP and the first one to retain the series' action-based gameplay. It is also the first canonical outing of the series for a portable platform, following the storyline of Metal Gear Solid 3: Snake Eater.

Set in 1970 in South America, six years after the events of Snake Eater, the game follows the exploits of Naked Snake after his former unit, FOX, goes renegade.The game also chronicles the eventual founding of FOXHOUND and The Patriots, as well as the inspiration of the military state Outer Heaven.

Unlike the previous titles on PSP, Metal Gear Acid and its sequel, which were turn-based tactical games with stealth elements, MPO retains the action-based gameplay from the console iterations, drawing heavily from Snake Eater and utilizing the camera system from Subsistence.

The main addition to MPO is the Comrade System. Instead of the solo missions that the series is known for, MPO goes for a squad-based approach, with Snake having to recruit allies and form a team of trained specialists. Before each mission, the player must compose a four-man squad. The squad is then sent into battle. Each member of Snake's squad has their own strengths and weaknesses. While some units are best utilized on the battlefield, others may specialize in producing items, healing allies, or providing intel for each of the game's maps.

During gameplay, the player controls only one squad member at a time. The squad members not in use hide themselves inside a cardboard box, and can be swapped into play when the player-controlled character finds a hiding spot, where they can hide in their own cardboard box.

Characters who are killed in combat are eliminated from the player's squad permanently, unless they are special characters, such as Snake. Special characters also include team mates that are bosses or supporting characters in the story. If a special character's health is reduced to zero, they are sent to the infirmary, in order to recover. The player can also abort the mission.

A variety of methods can be employed to expand one's squad. If an enemy character is tranquilized or stunned, they can be dragged to a waiting vehicle and captured. After a period of game time, the captured soldier will become a member of Snake's team. Also, enemy characters can be dragged to an ally waiting in a cardboard box, where, through the use of a transceiver frequency, they will be ordered to transport the enemy for the player, saving stamina. Alternatively, by accessing the PSP in certain hotspots using the system's Wi-Fi feature, soldiers and even special bonus characters can be recruited. The PSP GPS Receiver can also be used to similar effect. Because some of the player's recruits include former enemy soldiers or personnel, they can walk among the enemy undetected as long as the player avoids suspicious actions.

Another new feature is the surround indicator added to the game's HUD. Similar to the radar in previous titles, the surround indicator allows players to determine the relative proximity of enemy soldiers by the noises they make. The surround indicator is composed of two circles; the outer circle displays the noises made by enemies and inner circle displays noises made by the player himself.

The game also contains a Wi-Fi-enabled multiplayer mode, which is an expansion of the Metal Gear Online mode previously featured in Subsistence. One's performance in the Online Mode may affect their performance in the single player campaign; the player can recruit and trade soldiers from beaten opponents, or vice-versa. Additionally, certain multiplayer options result in recruits being removed from the one's single player roster permanently. In contrast to the console games in the series, the cutscenes that drive the story are not rendered using the usual in-game engine. Instead, they are presented using an animated comic style comprised of hand-drawn artwork by artist Ashley Wood. This style was previously utilized in Metal Gear Solid: Digital Graphic Novel. The game also features voice acting, consisting of returning cast members from Snake Eater and new actors. However, the number of cutscenes and in-game voiced sections are minimal, due to the PSP's UMD storage capacity constraints.