Emerson Palmieri will sign a four-year contract with the Hammers.
The Hammers have paid a fee in the region of £13 million for the Chelsea left-back who made only 71 appearances for the Blues.
The Italy international struggled to establish himself as a starter since his arrival from AS Roma in 2018.
He has won the UEFA European Championship, UEFA Champions League and UEFA Europa League in his professional career.
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“I’m very happy to be here and to arrive here,” said Emerson on the club’s website. “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.”