The 26-year-old joins Leeds in a deal worth £4.5m and the midfielder is immediately available to feature in Saturday’s Championship clash with Millwall at Elland Road.
Leeds entered the running to sign former Everton midfielder Forshaw after promotion rivals Cardiff moved to meet Boro’s valuation at the start of this week.
Cardiff boss Neil Warnock was intent on taking Forshaw to Wales and maintained his interest despite City signing another midfielder in Liverpool’s Marko Grujic on loan.
Forshaw, however, favoured a transfer to Leeds and United finalised a fee with Boro yesterday before the midfielder agreed personal terms and passed a medical in Leeds today.
Forshaw told the YEP: “I’m so excited.
“Obviously as soon as you get here there is the history and the size of the club and the fan base is huge.
“I just really can’t wait to get started now.”
Forshaw joins Leeds at a time when fellow centre midfielder Eunan O’Kane is banned for the next three games after being sent off in last weekend’s 1-0 loss at Ipswich Town.
Kalvin Phillips is also just one yellow card away from picking up an automatic two-match ban for receiving 10 cautions.
Ronaldo Vieira has been struggling with a knee problem but the teenager is expected to be fit for this weekend’s clash with Millwall.
Forshaw becomes United’s fifth new signing of the January transfer window, notably following swoops for Belgian left-back Laurens De Bock from Club Brugge and Japan international midfielder Yosuke Ideguchi from Gamba Osaka.
Ideguchi has joined Spanish Segunda Division side Cultural Leonesa on loan for the rest of the season to adapt to the European game ahead of Japan’s World Cup bid this summer.
The midfielder will then return to Elland Road to compete for a first team place next season.
Leeds have also signed young Finnish centre-back Aapo Halme from HJK Helsinki and Montenegro forward Oliver Sarkic, both of whom are currently playing for United’s under-23s.