The Italy left-back is West Ham's seventh summer signing, joining Nayef Aguerd, Gianluca Scamacca, Flynn Downes, Maxwel Cornet, Alphonse Areola and Thilo Kehrer in moving to the London Stadium.
Emerson spent four-and-a-half years at Chelsea after joining from AS Roma in January 2018 but he never cemented a regular starting berth at Stamford Bridge.
The 28-year-old spent the 2021/2022 season on loan at Olympique Lyon in the French Ligue 1.
Emerson won the Europa League and UEFA Champions League in his time at Chelsea.
He has won 27 caps for Italy and was part of the side that won the European Championship last year.
Speaking to West Ham's official website on his arrival at the club, Emerson said: "I'm very happy to be here and to arrive here. It's a big challenge for me, it's a big team, so I'm very happy to be here and I'm ready.
"Personally, since the first day I arrived here in England, I always saw the big clubs and I always watched the big games and I knew the history about West Ham.
"The history is so, so big for this team and so when I knew about the interest for sure I said 'Yes, let's go, I want to go there' and I'm here now! I can't wait to get started in a West Ham shirt."
West Ham United manager David Moyes added: "We are very pleased to welcome Emerson to West Ham United. He is an experienced player with a good pedigree, who has enjoyed great success at both club and international level in recent years.
"He will give us strong competition in defensive areas and brings a winning mentality that we are looking to build here at West Ham. We look forward to working with Emerson and wish him all the very best in his career with us."
West Ham are bottom of the Premier League after losing their first three games of the season.
