Fallout 3 is a 2008 action role-playing game developed by Bethesda Game Studios and published by Bethesda Softworks. The third major installment in the Fallout series, it is the first game to be developed by Bethesda after acquiring the rights to the franchise from Interplay Entertainment. The game marks a major shift in the series by using 3D graphics and real-time combat, replacing the 2D isometric graphics and turn-based combat of previous installments. It was released worldwide in October 2008 for Microsoft Windows, PlayStation 3, and Xbox 360.

The game is set within a post-apocalyptic open world environment that encompasses a scaled region consisting of the ruins of Washington, D.C., and much of the countryside to the north and west of it in Maryland and Virginia, collectively referred to as the Capital Wasteland. Downloadable content is set in Pennsylvania, pre-War Alaska and outer space. It takes place within Fallout's usual setting of a world that deviated into an alternate timeline thanks to atomic age technology, which eventually led to its devastation by a nuclear apocalypse in the year 2077 (referred to as the Great War), caused by a major international conflict between the United States and China over natural resources. The main story takes place in the year 2277. Players take control of an inhabitant of Vault 101, one of several underground shelters created before the Great War to protect around 1,000 humans from the nuclear fallout, who is forced to venture out into the Capital Wasteland to find their father after he disappears from the Vault under mysterious circumstances. They find themselves seeking to complete their father's work while fighting against the Enclave, the corrupt remnants of the former US government that seeks to use it for their own purposes.

Fallout 3 received a number of Game of the Year awards, and is considered one of the best video games ever made. Critics praised Fallout 3's open-ended gameplay and flexible character-leveling system, and the game shipped almost five million copies in its first week. It received post-launch support, with Bethesda releasing five downloadable add-ons. The game was met with controversy upon release in Australia, for the recreational drug use and the ability to be addicted to alcohol and other drugs; in India, for cultural and religious sentiments over the mutated cattle in the game being called Brahmin, a varna (class) in Hinduism; and in Japan, where a questline involving the potential detonation of a nuclear bomb in a prominent town was heavily altered. The game was followed by a spin-off, Fallout: New Vegas, developed by Obsidian Entertainment in 2010. The fourth major installment in the Fallout series, Fallout 4, was released in 2015.

Fallout 3 is an action role-playing game that can be played from either a first-person or third-person perspective. It is set in the Washington metropolitan area, years after a nuclear war left much of the United States decimated. The player controls a 19 year old character who grew up in a fallout shelter called Vault 101. The goal of the game is to complete a series of quests to find the character's father, who unexpectedly left Vault 101. In addition to the main quests, the player can participate in optional unrelated quests known as side quests. GamesRadar+ critic Andy Kelly estimates there are over 100 hours of content in Fallout 3.

At the beginning of the game, the player can customize their character's physical appearance by choosing their gender and race. They can then allocate points into seven primary attributes: strength, perception, endurance, charisma, intelligence, agility, and luck. These attributes are known as S.P.E.C.I.A.L. stats, and range from 1 to 10. Additionally, there are 13 secondary attributes whose point totals are affected by S.P.E.C.I.A.L. stats: barter, big guns, energy weapons, explosives, lockpick, medicine, melee weapons, repair, science, small guns, sneak, speech, and unarmed. If the player has a high charisma stat for example, then they will be more proficient with the barter and speech skills at the beginning of the game. The player can add more points into skill stats whenever they earn enough experience points to level up. Experience points can be earned through several methods, such as killing an enemy or completing a quest. When the player reaches a new level, they can select a perk, which is a permanent beneficial upgrade. For example, the perk Master Trader reduces the price of items sold by vendors by 25 percent.

Fallout 3 features an open world map that the player can freely explore. Locations the player can discover range from small settlements and abandoned buildings, to larger locations like the Jefferson Memorial and the Washington Monument. The player is equipped with a wearable computer called the Pip-Boy 3000. The device serves as a menu, and allows the player to access items they have acquired, view detailed character statistics and active quests, and look at the map. The player can use the Pip-Boy 3000 map to fast travel to previously discovered locations. The player can also use the Pip-Boy 3000 as a radio, and listen to songs from the 1940s and 1950s on makeshift radio broadcasts. While exploring, the player can recruit some non-playable characters as companions, who will accompany the player and assist them in combat. There a variety of weapons in the game, including standard guns, energy-based guns, melee weapons, and explosives. While in combat, the player can utilize a gameplay mechanic known as V.A.T.S., which pauses the game and allows the player to target specific body parts of an enemy. V.A.T.S. is dictated by a statistic known as Action Points. Each attack while in V.A.T.S. costs Action Points, and when the player runs out of Action Points they must wait a short period of time before they can use it again.

An important mechanic in Fallout 3 is the player's karma. Whenever the player commits an action that is deemed either good or bad, their karma will change accordingly. For example, if the player provides water to a beggar, their karma increases. Likewise, if the player breaks into a home, their karma decreases. The player's karma affects how other characters perceive them. Some companions can only be recruited if the player meets the companion's karmic expectation. Some non-recruitable characters will be more accepting of the player depending on their karma level. For example, slaver characters will be more accepting to players with negative karma, and provide services that would not be available to players with neutral or positive karma.

Fallout 3 takes place in the year 2277, 200 years after a nuclear war between the United States and China. Some survivors of the war took refuge in high-tech fallout shelters known as Vaults. The player character, nicknamed the Lone Wanderer, grew up in Vault 101, located near Washington, D.C. After the Lone Wanderer's 19th birthday, their father, a scientist named James (Liam Neeson), leaves the Vault without explanation. The young vault dweller decides to track down their father and journeys the region in and around Washington, D.C., now known as the Capital Wasteland. Along the way, the Lone Wanderer learns about their father's background from other characters, including Megaton resident Colin Moriarty (Mike Rosson), radio DJ Three Dog (Erik Dellums), and scientist Madison Li (Jennifer Massey). Before the Lone Wanderer's birth, James and his wife Catherine had been working on Project Purity, in which a water purifier built in the Jefferson Memorial would have purified the irradiated water in the Tidal Basin and Potomac River. However, Catherine died during childbirth, and James abandoned the project to raise his child in Vault 101.