Forshaw, 26, has put pen to paper on a four-and-a-half-year contract and will go straight into head coach Thomas Christiansen's squad for Saturday's home league game against Millwall.
"Obviously as soon as you get here there is the history and the size of the club and the fan base is huge".
Although the Dutchman could not move to another club having already played for NEC Nijmegen and Leeds this season, a return to the Netherlands with his former team on loan could be the best option for all parties.
Meanwhile, Boro's Leeds-born teenage winger Marcus Tavernier, who was propelled into the first-team limelight earlier this season, has joined League One side MK Dons on loan for the rest of the season.
Forshaw, who has found his opportunities limited this season and is yet to feature at all under Tony Pulis, will add some hardened Championship experience to a United midfield department which will be minus the services of the suspended Eunan O'Kane for the next three matches.
Toby, five, is suffering from neuroblastoma, a rare form of cancer which spreads rapidly across the body, and Leeds launched their One Day for Toby campaign in October.
"It is with great pride that Leeds United can announce that the funds for Toby Nye's Neuroblastoma treatment have been successfully raised", the club said.
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