The former England Under-21 winger, who has signed a contract until June 2014, steps straight into Roberto Mancini’s squad against Hull on Saturday. He will wear the traditional No.11.
Johnson, who played 19 games for England Under-21s and scored four goals, has played 96 times for Boro, scoring 13 times and making a firm impression on the game’s shrewdest judges.
He made his senior debut for Boro aged 17 in a UEFA Cup tie at Sporting Lisbon, playing his maiden Premier League match six months later in a 2-1 home win over Arsenal.
Loan spells of one month with Leeds in 2006 and three months the following year with Watford - where he scored five goals in 12 appearances - helped to speed up his development.
Manager Roberto Mancini said: “We are delighted that Adam has chosen to come here. He is a very good player with fantastic potential, and as a club we are always keen to sign British talent.”
