The 28-year-old joined Blackburn from school and was a key figure in their 1995 Premiership-winning side. Wilcox's arrival is a timely one for the Leeds manager, David O'Leary, who is struggling to contend with injuries to the midfield which includes the loss of both David Batty and David Hopkin.
The Scottish international Hopkin is out of the game until mid-January following groin surgery while the England international Batty is likely to remain unfit for another two weeks after a recurrence of a calf injury sustained in the Worthington Cup defeat to Leicester in midweek.
In addition, the England Under-21 midfielder Lee Bowyer has been playing all season with the possibility of a hernia operation looming.
Although Wilcox mainly plays on the left-hand side of midfield he is certain to become an integral squad member as O'Leary aims for silverware on three fronts. Leeds lead the Premiership, while they are also in the fourth round of both the Uefa and FA Cups.
The Leeds chairman, Peter Ridsdale, said: "This is a player we have admired for some time. He wasn't available when we first inquired about him but I would like to compliment Blackburn on the way they have handled the deal since. They have been impeccable.
"We are delighted that somebody of his calibre and experience has become available and that he wants to be part of what we are doing here at Leeds.
"I hope our supporters will recognise that yet again we have demonstrated our support for the manager when he has come to us and asked to acquire another player for the first-team squad."
Wilcox's pounds 3m fee depends on appearances and goals scored as well as Leeds' success.Reuse content