The 19-year-old reigning Football League young player of the year and arguably the hottest rising star in the Championship has signed a four-year deal with the Cherries after the south coast club agreed a deal with United.
The fee is believed to be an initial £6m, rising to £10m.
Cook left the Whites’ pre-season tour of Ireland to undergo a medical with Bournemouth yesterday, ahead of putting pen-to-paper with the top-flight outfit.
Cook’s exit is the latest in a increasing line of departures from United’s much-vaunted academy ranks over the past decade, with the likes of Sam Byram, Jonny Howson and Fabian Delph having previously left for pastures new.
While the blow of Cook’s departure has been slightly softened by the earlier £3m arrival of Oxford United forward Kemar Roofe, the disappearance of the club’s undisputed star turn, will be greeted with deep-seated disappointment by large sections of United’s support.
Cook, who featured 47 times in all competitions for United last season, progressed through the ranks at Thorp Arch to make his first-team debut at the start of the 2014-15 campaign.
The York-born player signed a new two-year deal at Elland Road last summer and was seemingly happy at Elland Road but despite the official party line being that United did not want to sell their midfield star, the club elected to cash in following Bournemouth’s significant offer.
Bournemouth chief executive Neill Blake said: “We are pleased to be bringing one of the country’s brightest prospects to the club.
“Lewis has impressed since earning his spot in the Leeds United first-team and we feel that he can continue his rapid progress here under the tutelage of Eddie (Howe) and Jason (Tindall).
“He’s represented England at youth level and won plenty of individual awards last season.”
Following Cook’s departure, United could firm up moves for former loan midfielder Liam Bridcutt, with the club also linked with ex-Watford defender Joel Ekstrand.
Meanwhile, Marco Silvestri’s career at the club looks over after he was told to train with the club’s Under-21 squad - having left United’s tour of Ireland.
The 26-year-old – Leeds’ first choice for the past two seasons – is surplus to requirements after the club signed England international and former West Ham goalkeeper Rob Green on Wednesday.