The Englishman has signed a four-year deal with the Mackems, ending a spell at Manchester City in which he struggled to truly break through into the first team
The 25-year-old has signed a four-year deal with the Mackems, and becomes the club's second signing on Friday following the acquisition of Steven Fletcher from Wolves.
Manager Martin O'Neill told the club's official website: “Adding quality players to the squad has been our main aim this summer and Adam certainly fits that bill.
“He has terrific ability, great delivery and I'm sure he is a player who will excite our fans. I couldn't be more delighted to have him at the football club.”
Johnson joined City in 2010 after several eye-catching performances for Middlesbrough following his graduation from the club's youth setup.
However, he failed to maintain a first-team spot throughout his time at the Etihad Stadium, regularly featuring as a substitute during Roberto Mancini's tenure.
The Middlesbrough-born wideman has 11 senior England caps but did not make Roy Hodgson's squad for Euro 2012 this summer.