Ashton said the chance to return to the Premiership was "too good to turn down" after the Hammers agreed a £7.25m deal with Norwich for the England Under-21 international.
West Ham will pay £7m plus a conditional £250,000, while a 15 per cent sell-on clause has also been negotiated. Ashton's former club, Crewe Alexandra, will receive £1.5m due to a sell-on clause included when they sold him to Norwich for £3m a year ago.
The West Ham director Scott Duxbury rejected suggestions that the club had paid too much money for Ashton.
Duxbury said: "West Ham regard Dean as a future England international and as such it's a good deal for the club. He has been compared to a young Alan Shearer and, at 22, we feel we may have signed the bargain of the season."
Ashton said: "I have had a great year at Norwich and would like to sincerely thank the fans and everyone I have worked with at Carrow Road. West Ham are a big club, I am ambitious and at the end of the day the chance of a swift step up to the Premiership is too good to turn down."
The Norwich chief executive Neil Doncaster said that a bid was received on Saturday and accepted after a meeting between the board and the Canaries manager Nigel Worthington following the defeat by Watford.
Worthington also believes that the deal is in the best interests of all parties. "We're always committed to trying to keep our best players at the club," he said. "But the size of the bid coupled with Dean's desire to make the move happen meant that after talking it through with the board and the lad himself we've decided to accept this offer."
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