Combat for the game was rebuilt, with fencing being used as an inspiration for the new system. In addition to returning weapons from previous games, Assassin's Creed Unity introduces the Phantom Blade. The Phantom Blade uses the mechanics of a crossbow to fire a silent projectile a great distance, while still serving the same role as the Hidden Blade in previous Assassin's Creed games. Navigation for the game was also overhauled. Ubisoft created new "Parkour Up" and "Parkour Down" mechanics to make it easier for the player to scale buildings in either direction. Additionally, Arno learns new parkour moves throughout the game, but the player can also purchase new skills as well. With the updated crowds, new interactions with them are also available. The crowd regularly presents many activities, appearing organically, that the player can choose to engage with at their leisure. Examples include interfering in a fistfight, or chasing down a pickpocket.

Assassin's Creed Unity also introduces cooperative multiplayer to the series. Players can enter taverns, which act as social hubs in the game, where you can see if any friends are playing Unity. If they are currently in a mission, they will appear as a "ghost" version of their player, allowing you to approach it to request to join their mission. If accepted, you are transitioned to their game and both of you reset to the most recent checkpoint, and continue on from there. Up to four players can join together in this fashion. In your own game, you’ll always be Arno – the other players will look like other random members of your brotherhood. Many missions and activities are available for cooperative play, but there are some story missions that are set aside to be single player only.