The club announced the news via Twitter on Saturday:
Per Stoke City's official website, Arnautovic will cost the Hammers at least £20 million, with £5 million in bonus money added to the deal. The fee is a club record.
The Austrian will be a huge miss for Stoke. He scored six goals and provided five assists in 32 Premier League appearances last season, per WhoScored.com.
However, he will bring creativity to a West Ham attack that managed only 47 goals in 38 Premier League games last season, as illustrated by Opta:
West Ham appear keen to strengthen their forward line this summer, having also been linked with a move for Arsenal striker Olivier Giroud and Bayer Leverkusen's Javier Hernandez, per Matt Law at the Telegraph.