West Ham have signed Austria forward Marko Arnautovic from Premier League counterparts Stoke City for a club-record fee worth up to £25million.
The Potters revealed West Ham will pay an initial £20m, with an additional £5m depending on certain clauses being met.
"It feels special to me to be a West Ham player," Arnautovic, who has signed a five-year deal, told West Ham's official website.
"Everyone knows that West Ham is a big club, with big history and I'm happy to be a part of it now. I can't wait to get started.
"I played against West Ham at the old stadium - I didn't play last year at the new stadium because I was suspended - but I came to the game and I can only say it is a massive club with a lot of fans.
"They are crazy for football and this is what I like. The club is still growing, getting better and better and that's why I'm here."
The 28-year-old former Inter player joined Stoke from Werder Bremen in 2013 and scored 26 times across 145 appearances for Mark Hughes' men.
Arnautovic signed a four-year contract extension at Stoke 12 months ago, but will now join fellow new recruits Pablo Zabaleta, Joe Hart and Javier Hernandez at London Stadium.
Hart and his former Manchester City team-mate are already in training with Slaven Bilic's squad, while Mexico international Hernandez is expected to undergo a medical on Monday to conclude a £13m move from Bayer Leverkusen.
"Alongside Pablo Zabaleta and Joe Hart we have brought in three players with vast Premier League experience this summer, and that was one of our key targets as we look ahead to the new season," said joint-chairman David Sullivan.
"Marko will bring plenty of quality to our front line and I'm excited to see him in a claret and blue shirt."