The Scottish striker, who scored 29 goals for Leeds last season, has signed a four-year contract at Craven Cottage.
The fee is a record paid by a second tier club in English football, eclipsing the £8m Blackburn spent on Jordan Rhodes last summer.
The move comes after Fulham asked 29 staff to take voluntary redundancy last week in a cost-cutting measure, a move designed to direct all available finances onto the pitch, which certainly seems to have happened in this move.
"The Scotland international, last season's top scorer in the Championship, has penned a four-year deal with the Whites, with the option of a further 12 months, potentially keeping him at the club until the summer of 2019."
Fulham had a £5m bid for McCormack turned down last month but have now succeeded with a much increased bid.
The reported £11m fee for McCormack makes him the second most expensive Scottish footballer of all time following Steven Fletcher's £12m move from Wolves to Sunderland in August 2012.
West Ham failed in their bid to sign McCormack in January, while Newcastle, former club Cardiff, Norwich and Derby had also been linked with the player.
Cardiff are understood to be owed around 15 per cent of the transfer fee due to a sell-on clause.
McCormack signed a new four-year contract at Elland Road last summer, but is understood to have asked for a transfer on June 20 - the day after David Hockaday had been appointed as the club's new head coach.
The transfer fee is more than had been expected for a player unproven in the top flight, but the deal may still anger some Leeds fans who had been hoping McCormack would spearhead their club's promotion challenge under Hockaday next season.
When Hockaday was unveiled by Leeds president Massimo Cellino on June 19, the Italian insisted McCormack would only be sold for the right price to a Barclays Premier League club.
Cellino said he had blocked the Scot's proposed move to Fulham's London rivals West Ham in January. He also claimed he rejected an approach from Newcastle in May.
But McCormack is reported to have told Cellino he wanted to leave the club in a face-to-face meeting at the end of last month and the former Cardiff forward did not travel with the squad last week to its summer training camp in Italy.
McCormack, who started out at Rangers, spent two seasons at Motherwell before joining Cardiff on a free transfer in June 2008.
Leeds paid an undisclosed fee, believed to be around £350,000, for McCormack in the summer of 2010 and after a slow start to his spell at Elland Road he became the club's main striker under Neil Warnock.
He flourished last season after being installed as captain by previous boss Brian McDermott, scoring a total of 29 goals in 53 appearances.
Fulham's summer transfer kitty is boosted by the Premier League's parachute payment following relegation last season and boss Felix Magath had made McCormack his number one target.
He becomes the Cottagers' fifth summer signing following the arrivals of Tim Hoogland, Adam Taggart, Kay Voser and Shaun Hutchinson.
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- Ross McCormack