British media reported that West Ham paid a fee in the region of 15 million pounds for Emerson, who was part of the Italy team that lifted the European Championship last year.
He moved to Chelsea from AS Roma in 2018, but could never establish himself as a regular at Stamford Bridge and spent last season on loan at French outfit Olympique Lyonnais. He made 71 appearances in all competitions for Chelsea.
"It's a big challenge for me, it's a big team, so I'm very happy to be here and I'm ready," Emerson told the West Ham website.
Emerson becomes West Ham's seventh new signing and joins them with David Moyes's side sitting bottom of the league table following defeats in their opening three games.
"He is an experienced player with a good pedigree...he will give us strong competition in defensive areas and brings a winning mentality that we are looking to build here at West Ham," said Moyes.
West Ham travel to Danish side Viborg for the second leg of their Europa Conference League playoff on Thursday. They won the home leg 3-1.
