West Ham have signed Andriy Yarmolenko from Borussia Dortmund.
The Premier League club agreed a £17.5m deal with Dortmund earlier this week, Sky Sports News understands.
Yarmolenko then underwent a medical at West Ham's training camp in Switzerland earlier this week before signing a four-year contract.
"West Ham is a big club with good fans and I am happy to come to play in the Premier League," Yarmolenko said.
"The Premier League is the best league in the world and I know that an interesting project is being built here at West Ham. The team wants to achieve high things and I am excited for this challenge."
The 28-year-old is Manuel Pellegrini's fifth signing since taking over as West Ham manager.
The club have recruited Jack Wilshere (free), Ryan Fredericks (free), Lukasz Fabianski (£7m, Swansea) and Issa Diop (£22m, Toulouse).
West Ham's director of football, Mario Husillos, said the club "did not hesitate" when the opportunity to sign Yarmolenko came about.
"He really is the kind of player that we were looking for," said Husillos.
"He is a left-footed player but can play on the right and is very quick, very good one-on-one against a defender. He gives the team a lot of attacking options and scores goals, is fast on the counter-attack and provides aerial threat with his height.
"He also has very good experience, of playing for big teams and fighting for titles. At Dortmund, he had some good matches but perhaps didn't quite show his very best. However, I think Manuel Pellegrini is the perfect coach for Andriy and his style of play.
"I believe he is joining us in the perfect moment. He is 28, the prime age for football, and we have great expectation about what he can achieve with West Ham United."