West Ham have completed the signing of Nikola Vlasic from CSKA Moscow.
The fee for Vlasic is 30m euros (£25.7m) plus 9m euros (£7.7m) in add-ons, with the contract running until 2026 and also including a one-year option.
West Ham manager David Moyes told the club's official website: "I am delighted to welcome Nikola to West Ham United. I have been watching Nikola improve over recent seasons - he has got a good goals and assists record during that time.
"He is also a Croatia international with experience and still being only 23, I feel there is more to come from him.
"I was really impressed with how determined Nikola was to become a West Ham player. It's that hunger and ambition that fits with what we are trying to build here at the club."
David Moyes has been keen to bolster his attacking options and has turned his attention to the Croatia international after it became increasingly likely Jesse Lingard would remain at Manchester United this season.
If Manchester United were to receive an offer in the final hours, it will be discussed with Lingard and a collective decision will be made.
Lingard excelled during a successful loan spell at West Ham under Moyes last term, with the 28-year-old's performances reinstating him into the thoughts of England boss Gareth Southgate.
Vlasic has previously had an underwhelming two-year spell at Everton which came to an end in 2019.
The 23-year-old joined Everton on Deadline Day in the summer of 2017 for a fee of around £8m from Hadjuk Split but only managed 19 appearances during his spell with the club.
He initially impressed Everton boss at the time Ronald Koeman, but struggled for game time under Sam Allardyce who replaced the outgoing Dutchman in November 2017.
Vlasic, who plays as a No 10, was part of Croatia's squad at Euro 2020 and scored the opening goal in the 3-1 win over Scotland which ensured qualification for the round of 16.