The 28-year-old could make his debut for Leeds against the Bees on Tuesday.
Diagouraga joined the Griffin Park side in July 2010 and made 190 league appearances for the club.
Leeds head coach Steve Evans told BBC Radio Leeds: "We're looking forward to his contribution not only tomorrow but for over many months and years."
Scottish Championship leaders Rangers had wanted to sign Diagouragega, but a deal could not be concluded with Brentford.
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Leeds United have signed French midfielder Toumani Diagouraga from Brentford for an undisclosed fee on a two-and-a-half-year contract.