The 24-year-old moves to north London on a four-year contract for an undisclosed fee.
And Koscielny jumped at the chance to link up with his countryman for the chance to bolster a defence which conceded 41 league goals last term.
"It wasn't a difficult decision to make. It is exceptional to have a club like Arsenal wanting to sign you," Koscielny told the club's official website.
"I don't believe any French player would have refused to come and play for Arsenal. It is a very big club that plays in the Champions League and it is always near the top of the Premier League.
"Arsenal is a reference for football and we all know the quality of their game. The decision was quickly taken - I took the decision the day after hearing about their interest in me.
"I know the club have the means and the ambition to get that Premier League title and to go as far as possible in the Champions League."