Assassin's Creed: Brotherhood is a 2010 action-adventure game developed by Ubisoft Montreal and published by Ubisoft. It is the third major installment in the Assassin's Creed series, and the second chapter in the "Ezio Trilogy", as a direct sequel to 2009's Assassin's Creed II. It was released on the PlayStation 3 and Xbox 360 in November 2010, and on Microsoft Windows in March 2011. A remastered version of Brotherhood, along with Assassins's Creed II and Assassin's Creed: Revelations, was released as part of The Ezio Collection compilation for the PlayStation 4 and Xbox One on November 15, 2016, and for the Nintendo Switch on February 17, 2022.

The game's plot is set in a fictional history of real-world events and follows the millennia-old struggle between the Assassins, who fight to preserve peace and free will, and the Templars, who desire peace through control. The framing story is set in the 21st century and features series protagonist Desmond Miles using a machine known as the Animus to relive the memories of his Assassin ancestor, Ezio Auditore da Firenze, and find a way to avert the 2012 apocalypse. The main narrative takes place during the Italian Wars, spanning the years 1500–1507, and continues from the events of Assassin's Creed II, as Ezio takes the fight against the Templars (led by the powerful Borgia family) to Rome, where he attempts to rebuild the Assassin Brotherhood in Italy and liberate the city from the Borgias' control.

Brotherhood features an open world and is played from the third-person perspective, with a primary focus on using Desmond's and Ezio's combat, climbing and stealth abilities to eliminate targets and explore the environment. As Ezio, players can freely explore Rome to advance the narrative, or complete a variety of side missions unrelated to the main storyline. The game introduces a multiplayer component to the series, in which players assume the role of Templars in training. A number of downloadable content (DLC) packs were released to support Brotherhood, including The Da Vinci Disappearance, a story expansion set during the events of the single-player campaign.

Brotherhood received critical acclaim, with praise directed at its setting, new content, gameplay improvements over its predecessor and the new multiplayer mode. The narrative was also positively received, although it was generally seen as inferior to that of Assassin's Creed II. The game won multiple awards including a BAFTA award for Best Action Game. It was also commercially successful, shipping 7.2 million units by May 2011. The final installment in the Ezio Trilogy, Assassin's Creed: Revelations, was released in November 2011.

Assassin's Creed: Brotherhood is an action-adventure, stealth game set in an open world environment and played from a third-person perspective. The game features both single-player and multiplayer modes, a first for the series. For most of the single-player campaign, which is estimated to provide 15+ hours of gameplay, the player controls returning protagonist Ezio Auditore da Firenze from Assassin's Creed II; at predetermined points in the narrative, the player takes control of Desmond Miles, who is reliving Ezio's memories through the Animus in the modern-day. For the first time in the series, Desmond can leave the Animus at almost any given time to explore Monteriggioni, where the Assassins have set up base, and find hidden secrets. Desmond can also access virtual training missions via the Animus, which test the player's skills in combat and free-running.

The series' combat system has been modified in Brotherhood. Striking first and offensive actions are more deadly than in previous games where counter-attacks were the most efficient. Before, this made the player wait until their AI opponents struck, which slowed down the pace of fighting. The AI in this game is thus more aggressive and enemies can attack simultaneously. To dispatch them, Ezio can use melee and ranged weapons at the same time, including his hidden pistol, which could previously not be used in combat. After killing one enemy the player can start an execution streak to dispatch multiple enemies quickly. Ezio can also throw heavy weapons (axes, spears, and swords) at his enemies. The variety of enemies also saw an increase with the introduction of new archetypes such as horsemen, arquebusiers, and papal guards.

The game introduces the titular Brotherhood system, which allows the player, as Ezio, to recruit new Assassin initiates after destroying any of the twelve "Borgia towers" around Rome where Papal troops are stationed, and then rescuing disgruntled citizens from being harassed by city guards. The player can send these Assassin recruits on assignments around Europe or call them for support during missions (if they are not already occupied). Tasking the novice Assassins makes them gain experience, and the player can customize their appearance, skills, and weapon training to some degree by spending the skill points they have earned. Assassins can die on missions, from which they will not return.

In Brotherhood, Ezio gets to master several new gadgets, including Leonardo da Vinci's parachute, which can be used when jumping from tall buildings, as well as poison darts, a faster acting poison, and a crossbow. A series of side missions given by Leonardo allow the player to briefly use his war machines, such as a cannon-equipped Great Kite and a prototype tank, which he was forced to design for the Templars; Ezio must destroy these machines and their designs before more can be created. Other side missions include assassinating Templar agents in Rome, completing challenges for the three ally factions in the game (Courtesans, Thieves, and Mercenaries), and exploring underground locations to find the keys to the treasure of the Followers of Romulus—a secret cult who worship Romulus, the legendary founder of Rome. After reaching certain levels of synchronization, the player can also relive several repressed memories of Ezio's past lover, Cristina Vespucci.