The 24-year-old has been making headlines for several months now, having also gained interest from City's cross-town rivals Manchester Untied
, who had a £20 million bid rejected by the Gunners last month.
The news comes as another blow to Arsenal, who have also lost Gaël Clichy
to Manchester City, as well as former captain Cesc Fàbregas
City's summer spending has now surpassed the £75 million
mark, as they look to build on last season's third-place finish in the Barclays Premier League.
Roberto Mancini told the club's official website:
"Samir is a fantastic player with a great technique and mentality.
"I have followed him since he was at Marseille and I wanted to bring him to Inter Milan five years ago.
"He has improved over his four years in the Premier League with Arsenal and he is a top player.
"He can score goals and make assists and he is a similar player to David Silva - but they can both play in the same team."
Nasri joined Arsenal from Marseille in 2008, and although he never won a trophy with the Gunners, he claimed many individual honours.
The Frenchman - who may debut against Tottenham on Sunday - will join up with City tomorrow.