Dortmund also took to their social media outlets to confirm the deal, while wishing the player good luck for his future endeavors in the Premier League.
Yarmolenko, 28, has joined in a £17.5m switch from Bundesliga outfit Borussia Dortmund. "He has signed a four-year deal".
Goal understands West Ham have tabled a club-record £40 million (€45m/$52m) bid for the versatile attacking midfielder, and remain in talks with the Serie A club.
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Perez was rushed to Cooper University Hospital in Camden, where he was quickly admitted into the intensive care unit. With the prognosis still unknown, the Perez family is praying Angel leaves the hospital with his life and limbs.
"We are very happy to welcome Andriy", West Ham director of football Mario Husillos said in a statement.
"Jack also knows the Premier League very well, and I know all our supporters will join me in welcoming them both to the London Stadium".
The team wants to achieve high things and I am excited for this challenge.
Sources have also confirmed that central defender Fabian Balbuena, now of Brazilian club Corinthians, is set to be unveiled imminently; West Ham have triggered the Paraguay international's release clause.