Assassin's Creed Syndicate is a 2015 action-adventure game developed by Ubisoft Quebec and published by Ubisoft. It was released on October 23, 2015, for PlayStation 4 and Xbox One, and on November 19, 2015, for Windows. It is the ninth major installment in the Assassin's Creed series, and the successor to 2014's Assassin's Creed Unity.

The game's plot follows the premise of the Assassin's Creed franchise, involving a fictional history of real-world events where a secret war has been fought for centuries between two factions: the Assassins, who promote peace and liberty, and the Templars, who desire peace through control. The framing story is set in the 21st century and features the same unnamed and unseen protagonist from Assassin's Creed Unity who assists the Assassins in their race against the Templars to find an artifact hidden in London. The main story is set in London in 1868, at the onset of the Second Industrial Revolution, and follows twin Assassins Jacob and Evie Frye as they navigate the corridors of organised crime and take back the city from Templar control. The game also includes segments set in 1916, during World War I, which follows Jacob's granddaughter, Lydia Frye.

The game is played from a third-person perspective and its open world is navigated on foot or by carriage. Syndicate introduces new travelling systems to the series, as well as refined combat and stealth mechanics. Players control the two lead characters throughout the main story, switching between them both during and outside of missions. After launch, the game was supported with several releases of downloadable content (DLC), including three story expansions. The most notable of these, Jack the Ripper, is set twenty years after the main campaign and involves Jacob's pursuit of the titular unidentified serial killer.

Assassin's Creed Syndicate received positive reviews upon release, with praise for its visuals, characters, narrative, and level design, while the combat, open-world design and vehicle gameplay were met with some criticism. The game was nominated for multiple awards, including Best Action/Adventure at The Game Awards 2015. Syndicate was less financially successful than previous entries in the series, selling over 5.5 million units by November 2017. Ubisoft attributed the lower sales numbers to a series fatigue among players, caused in part by Assassin's Creed Unity's disappointing release the year prior, and decided to end the annual release cycle for the series. The next main entry, Assassin's Creed Origins, primarily set in Ptolemaic era Ancient Egypt, was released in October 2017, and acts as a soft reboot which introduces more role-playing mechanics and a new modern-day storyline.

Assassin's Creed Syndicate is an action-adventure, stealth game played from a third-person perspective, that features similar gameplay elements to the previous game Assassin's Creed Unity. Players complete missions—linear scenarios with set objectives—to progress through the story. Outside of missions, the player can freely roam the open world. Composed of the greater area of Victorian London, consisting of seven boroughs, the world of Assassin's Creed Syndicate is larger than previous entries in the series. In keeping with a historical context that more closely resembles the modern-day, the city guard of previous iterations is replaced by a Victorian-era police force, who will rarely attack the player unless a crime is committed in their presence; the player's main enemy is instead a Templar-controlled street gang called the "Blighters".

The game lets the player control two characters: twins Jacob and Evie Frye. Jacob is a hot-headed brawler, specialising in close-ranged combat, while Evie is strong in stealth and relies on her intelligence and wit. Evie is notably the first female playable character in a mainline Assassin's Creed game. In contrast to the previous entries' swords and long weapons, the main weapons of Syndicate include era-accurate brass knuckles, compact revolvers, cane-swords, and traditional Nepalese curved kukri knives, all of which can be upgraded to increase their effectiveness. The twins can also be equipped with various outfits and clothing items (belts for Jacob and capes for Evie) which present different statistics, as well as gain additional abilities through a skill tree system. As the player progresses through the game, they earn experience points to level up their characters; for every 1,000 experience points earned, they unlock a "skill point" which can be spent on a new ability. The number of skills unlocked determines each twin's level, which can be a maximum of 10. Enemies also present levels, determining how difficult they are to defeat in combat. However, all enemies can be stealthily assassinated, regardless of level.

The game's combat system was changed from Assassin's Creed Unity, and now focuses on chaining together fast-paced attacks and blocks. The stealth system also saw several improvements with the addition of new gadgets and a kidnapping mechanic, which allows the player to take an enemy hostage and blend in with them to remain unseen, as long as they are not in close proximity to another enemy. Syndicate also builds upon Unity's "Black Box" design for assassination missions by introducing more opportunities for infiltration and distraction, as well as "cinematic kills", which are unlocked after certain parameters are met. For navigation, the game adds several new systems, including a rope launcher, which allows the player to rappel up structures, or create a zip-line between buildings; carriages, which can be piloted or simply occupied by the player, and can be the setting of fights and parkour chases; and a train, which serves as the main base for the player throughout the game.

The side content in Syndicate was designed to reflect the fight for power in London, and is cohesive to the game's main story, adopting a gang warfare mechanic where the city is divided into districts each controlled by the game's two main street gangs. During the game, the Frye twins start a street gang called the Rooks to combat the Blighters' influence, which can be leveled up with various upgrades pertaining to their weaponry, vehicles, and economic opportunities, increasing their effectiveness in combat as well as the passive revenue generated by the gang over time. At the start of the game, London is controlled entirely by the Blighters, but as the player completes optional liberation missions (which can range from freeing child labourers to kidnapping or assassinating a high-profile Templar), each borough slowly comes under the control of the Rooks, culminating in a gang fight to definitively take over the borough. Other side missions focus on assisting prominent historical figures such as Charles Dickens, Karl Marx, and Queen Victoria. Players can also engage in a number of smaller activities for monetary rewards, like fight clubs and carriage races, and find several different types of collectibles, including music boxes containing the keys to a hidden treasure.