Which Halo game was the first to include a playable campaign for four-player online co-op?

Halo 5: Guardians was the first Halo game to include a four-player online cooperative campaign.

Released for Xbox One in 2015, Halo 5 built its campaign around two linked fireteams: Blue Team, led by Master Chief, and Fireteam Osiris, led by Jameson Locke. A single player could use AI-controlled teammates, while up to four players could cooperate online across the campaign’s missions.

This was a major structural change from earlier Halo games. Halo 3 and Halo: Reach supported cooperative campaign play, but their campaign co-op formats did not provide Halo 5’s four-player online fireteam structure. Halo 5 also removed split-screen campaign co-op, making online connectivity essential for a full human team.

Halo Infinite later returned to a different cooperative design, adding online campaign co-op after launch and offering local split-screen plans that changed during development. Those later features do not alter Halo 5’s place as the first four-player online campaign entry.