Which Assassin's Creed game was the first in the series to include a playable multiplayer mode?

Assassin's Creed Brotherhood was the first Assassin's Creed game to include a playable multiplayer mode.

Released in 2010, Brotherhood added competitive online multiplayer alongside its single-player campaign. Players controlled trained characters in modes built around stealth, disguise, pursuit, and assassination rather than conventional team-based shooting.

The single-player story continues Ezio Auditore's struggle against the Borgia family in Renaissance Rome. It also expands the Brotherhood-management system, allowing Ezio to recruit and train Assassins who can support missions across Europe.

The series had included online-connected features before Brotherhood, but those are not the same as a full playable multiplayer mode. Later entries expanded or changed multiplayer, while some games focused entirely on single-player experiences. This distinction makes Brotherhood the specific answer rather than Assassin's Creed II or Revelations, which are often named because they are adjacent entries in Ezio's story.