Vincent Janssen scored 27 goals in 34 league appearances for AZ Alkmaar last season.
26-year-old becomes the club's second summer arrival as Slaven Bilic chases Gokhan Tore and Atiba Hutchinson.
French midfielder scored the hosts' winning goal in the opening game of Euro 2016.
Full-back Joey O'Brien has also left the club after his contract expired.
Jack Sullivan, son of co-owner David, says club are not pursuing the England internationals.
West Ham United chairman David Sullivan has admitted that the club are awaiting responses on five bids.
The 27-year-old Englishman struggled for first-team opportunities towards the end of the season.
Co-owner David Gold has already confirmed that £25m-£30m will be spent on bolstering forward unit this summer.
Alexandre Lacazette has been impressive for Lyon in the last two campaigns and has admitted interest in PL move.
Reece Oxford has two years left on his contract at West Ham United.
The east London club have already had bids for Alexandre Lacazette and Callum Wilson rejected.
Michael Carrick is out of contract at Manchester United on 30 June and is hoping to extend his stay at Old Trafford.
Quina has chosen to join West Ham United on a two-year scholarship after leaving the Chelsea youth setup.
Cherries boss confirmed £25m offer, received before Manchester United game, has been rejected.
Hammers reportedly had a club record £31m bid for French striker rejected by Lyon.
West Ham will demand a minimum of £10m to allow Oxford to leave the club.
The versatile Norwegian will return to English football on 1 July after leaving Borussia Monchengladbach.
Scotland Yard said the 28-year-old handed himself into an East London police station.
Caretaker boss Franck Passi has confirmed that coveted Belgian striker will be sold.
Hammers' move to the Olympic Stadium underlines football's dependence on revenue.
Jean Michel Aulas says Lacazette is 80% likely to stay beyond the summer transfer window.
Sullivan was initially criticised for suggesting that no such attack took place and that no damage had occurred.