The Italy Under-21 striker completed a medical at Fulham's training ground last week and the club confirmed the signing in a statement on their website on Monday morning.
The loan deal appeared to be in doubt two weeks ago when Okaka picked up an injury but Fulham manager Roy Hodgson stepped up his pursuit of the striker after Andy Johnson and Clint Dempsey both picked up knocks in recent weeks.
Loan ranger: Roma forward Okaka (left) in action against Siena
Bobby Zamora, who has only just recovered from a collarbone injury, led the line with Erik Nevland against Aston Villa on Saturday while inexperienced Swede David Elm is Hodgson's only other recognised striker after Diomansy Kamara left for Celtic on deadline day.
Okaka announced he was moving to London after scoring a cleverly backheeled winner in Roma's 2-1 win over Siena on Sunday afternoon.The 20 year old, who's full name is Stefano Okaka Chuka, came through the youth ranks at Roma and has made seven league appearances for Claudio Ranieri's side this term, all as a substitute.
He started both of this season's Europa League matches between the clubs and scored the winner at the Stadio Olimpico in early November.
Treatment table: Andy Johnson (left) and Clint Dempsey (right)
The deal could be extended by 12 months if both parties agree and Okaka believes a loan spell in West London will be crucial for his development as a player.
'I have been with Roma for five years,' said Okaka. 'I have grown thanks to the advice of my team-mates.
'But to have a leap in quality, it is crucial for me to go to England and play in the Premier League. I hope I can return stronger than before.
'The loan deal expires in June but there is the option to remain there for a further year. We shall see.'
The powerful striker has also had loan spells at Serie B sides Modena and Brescia over the last two years and could go into Hodgson's squad for Tuesday's match with Barclays Premier League bottom side Portsmouth.
Ranieri expects Okaka to return to Rome an improved player.
'He has been at Roma for five years and I believe the experience in England could mark a turning point in his career,' the former Chelsea boss said. 'He can do a lot more and the Premier League will give us back a much stronger player.'
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