The 27-year-old featured in 44 games across all competitions for Chelsea in his debut 2023-24 season, but in his third campaign with the club he has not featured once.
Speaking about his arrival Disasi said: "I'm very happy to be here, to have this opportunity," Disasi told West Ham's club media. "I'm just ready to fight and to play games."
Disasi joined Aston Villa on loan during the winter transfer window last season and featured 10 times during his time with Unai Emery's side.
He added: "Everyone knows my situation in the last few months, so I just want to get back on the field, feel the sensation of games and help the team, he added.”
"The club has given me the opportunity to show my quality on the pitch. I spoke with all the people here, and I feel that they really wanted me, so that's why I'm here today."
"It's an exciting challenge. We know that the club is not at the place that it should be, but I'm here to help the team and to bring what I can to achieve our goal."