The 29-year old has been snapped up by the Gunners on a free transfer after he departed AC Milan earlier this summer.
Flamini made 153 appearances in all competitions for Arsenal between 2004 and 2008 and now returns to the Emirates Stadium after spending the past five seasons with Milan.
"We are very pleased Mathieu has agreed to rejoin us," Gunners boss Arsene Wenger told arsenal.com.
"He is a player of real quality, with a fantastic work rate and great mentality.
"As we know, Mathieu is a strong midfielder, but is also is comfortable playing in defence too. He is a very good addition to our squad."