The 44-year-old spent six years at Middlesbrough and was part of the backroom staff at the Riverside Stadium until the end of last season.
He left to take up the role of assistant manager at nearby Hartlepool United, but has now agreed a return to Middlesbrough.
"Boro have tonight confirmed the appointment of club legend Craig Hignett as assistant head coach to Aitor Karanka," read a statement on Middlebrough's official website.
"Hignett joins from neighbours Hartlepool United, (and) brings a wealth of experience."
Hartlepool also confirmed the news and stated Hignett would not work his notice period to enable him to start his new role on Tuesday.
Jamie Clapham, who had assisted Karanka at Middlesbrough since the Spaniard's arrival in November, will now return to his previous role with the club's Under-21 side.
