The Congolese forward was Forest's top scorer last season with 14 goals and managed to overcome a serious knee injury to become a key player.
Premier League sides Burnley and Watford lost the battle to sign Assombalonga due the former being unwilling to meet Forest's offer for their star striker, while the latter pulled out upon increased bidding.
With such spending power being demonstrated by Boro, it is little wonder that Chariman Steve Gisbson was able to attract former Swansea and Leeds boss Gary Monk to The Riverside. Monk left Swansea in 2015 and quit Leeds in May before joining Middlesborough.
An immediate return to the Premier League is among Monk's top priorities for his new side and this signing will be a step in that direction when Boro kick off their Championship campaign against Wolves on 5th August.