Metal Gear Solid 2: Sons of Liberty is a 2001 action-adventure stealth game developed by Konami Computer Entertainment Japan and published by Konami for the PlayStation 2. It is the fourth Metal Gear game produced by Hideo Kojima, the seventh overall game in the series, and a sequel to Metal Gear Solid (1998). The game was originally released on November 13, 2001, while an expanded edition, titled Metal Gear Solid 2: Substance, was released the following year for the Xbox and Windows, in addition to the PlayStation 2. A remastered version of the game, Metal Gear Solid 2: Sons of Liberty - HD Edition, was later included in the Metal Gear Solid HD Collection for the PlayStation 3, Xbox 360, and PlayStation Vita. The HD Edition of the game was included in the Metal Gear Solid: Master Collection Vol. 1 compilation for Nintendo Switch, PlayStation 4, PlayStation 5, Windows, and Xbox Series X/S, which was released on October 24, 2023.

The story revolves around the Big Shell, a massive offshore clean-up facility seized by a group of terrorists who call themselves the Sons of Liberty. They demand an enormous ransom in exchange for the life of the President of the United States and threaten to destroy the facility and create a cataclysmic environmental disaster if their demands are not met. The motives and identities of many of the antagonists and allies change throughout the game, as the protagonists discover a world-shaking conspiracy constructed by a powerful organization known as the Patriots.

Metal Gear Solid 2 received acclaim for its gameplay, graphics, and attention to detail. However, critics were initially divided on the protagonist and the philosophical nature and execution of the game's storyline, which explores many themes, such as memetics, social engineering, artificial intelligence, virtual reality, and the internal struggle of freedom of thought. The game was a commercial success, selling seven million copies by 2004. It has since been considered to be one of the greatest video games of all time, as well as a leading example of artistic expression in video games. The game is often considered ahead of its time for dealing with themes and concepts such as post-truth politics, fake news, alternative facts, synthetic media, and echo chambers, that became culturally relevant in the mid-to-late 2010s.

Metal Gear Solid 2 carries the subtitle of "Tactical Espionage Action," and most of the game involves the protagonist sneaking around to avoid being seen by the enemies. Most fundamental is the broader range of skills offered to the player. The first-person aiming mode allows players to target specific points in the game, greatly expanding tactical options; guards can be blinded by steam, distracted by a flying piece of fruit or hit in weak spots. Players can walk slowly, allowing them to sneak over noisy flooring without making a sound, or hang off walkways to slip past below guards' feet. The corner-press move from Metal Gear Solid, which allowed players a sneak peek around the next bend, is expanded to allow players to fire from cover. Other abilities included leaping over and hanging off of railings, opening and hiding in storage lockers, and sneaking up behind enemies to hold them at gunpoint for items and ammunition. Players can shoot out the enemy's radio, so they are unable to communicate with others on their team. The environment also has a more significant impact on the stealth gameplay, taking into account factors such as weather, smell, atmosphere, and temperature.

In Metal Gear Solid 2, the enemy guards are given more advanced AI "to prevent an imbalance of power", and unlike the original Metal Gear Solid, work in squads. They call on their radios for a strike team upon seeing the player, then attempt to flank him and cut off his escape while avoiding the player's attacks. Often strike teams will carry body armor and riot shields, making them a greater threat. Even if the player escapes to a hiding place, a team will sweep in to check the area. The game has a collective enemy AI, where enemy guards work together in squads, can communicate with one another, and react more realistically towards the player. The game's enemy AI was considered one of the best in gaming for many years.

The game expanded its predecessor's cover mechanic, with Solid Snake or Raiden able to take cover behind walls or objects and pop out to shoot at enemies, while the improved enemy AI allowed enemies to also take cover from the player character. The enemies will often take cover to call for backup, but during battle, they will take cover then pop out and shoot at the player or blindly throw grenades from behind their cover. The game features a laser sight mechanic, where a laser sight helps assist with manually aiming a gun. Boss battles and set-pieces remain a case of finding a strategy that bypasses the defenses of the enemy. However, in a major break from action game standards, it is also possible to clear the entire game, including boss fights, without causing a single deliberate death, through use of tranquilizer guns, stun grenades, and melee attacks.

Sons of Liberty takes place within an alternate history of events, in which the Cold War ended during the late 1990s. The game's story focuses on two narratives that occur around and near New York City  – the first in 2007, two years after the events of Metal Gear Solid; and the second in 2009. The game's plot is the fourth chapter of an overarching plot concerning the character of Solid Snake.

The main protagonist of Metal Gear Solid 2 is a young rookie agent named Raiden. He is supported by his commanding officer, the Colonel, and Rosemary, his girlfriend. Allies formed on the mission include Lt. Junior Grade Iroquois Pliskin, a Navy SEAL of mysterious background who provides his knowledge of the facility; Peter Stillman, an NYPD bomb-disposal expert; Otacon, a computer security specialist and friend of Solid Snake; and a cyborg ninja imitating Gray Fox's persona, first calling itself Deepthroat, then changing its name to Mr. X.