The 19-year-old winger completed his move this afternoon after passing a medical yesterday.
The deal was sealed after the squad left London for Wednesday's Barclays Premier League game at Liverpool, while Kakuta is also cup-tied for Sunday's FA Cup fourth-round clash with Tottenham.
True Blue: Gael Kakuta is one of Chelsea's most promising young players
He therefore would not be able to make his debut until next Tuesday's league game with Newcastle at Craven Cottage.
Fulham boss Mark Hughes last week revealed his interest in19-year-old winger Kakuta, who signed a new four-and-a-half-yearcontract at Chelsea last month.
Hughes, who was also eyeing Manchester City winger ShaunWright-Phillips, said on Friday: 'He's a player I know and I like thequalities he has.'
Controversy: Kakuta was thrust into the limelight following his move to Stamford Bridge from RC Lens
He becomes Hughes' second January signing following the permanent capture of Steve Sidwell from Aston Villa.
France Under 20 international Kakuta was widely regarded as one ofthe brightest prospects in the game when he joined Chelsea from Lens in2007.
He has made 16 appearances for the Blues but only five starts and was an unused substitute in Monday night's 4-0 win at Bolton.
He was at the centre of a major row in 2009 when he was accused of breaking his contract with Lens in order to join Chelsea.
He was subsequently banned for four months and given a fine of more than £670,000 by FIFA.
Chelsea were also fined and banned from any activity during twotransfer windows, but the club appealed, claiming Kakuta did not have avalid contract with Lens.
The sanctions were lifted by the Court of Arbitration for Sport in February last year.
Hot prospect: Young French ace Kakuta is highly-rated at Stamford Bridge
Kakuta has played for France at every level from Under 16 to Under20 and in December 2009, aged 18, became the youngest ever player torepresent Chelsea in the Champions League.
That came after he suffered a fractured ankle which had kept him out of action for six months.
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