"Koscielny is a central defender with great ability who performed extremely well last season for Lorient," Arsenal manager Arsene Wenger said on the club's website.
"We identified him as a very strong centre half, who has made big progress very quickly. He has shown he is mentally strong, he's a fighter and a very strong competitor. Koscielny is a great addition to our squad."
The 24-year-old Koscielny, who would also qualify to play for Poland, will wear the number six shirt for Arsenal.
"I'm very excited to be joining Arsenal. They are one of the biggest teams in Europe with many great players," Koscielny said. "I'm really looking forward to playing in the English Premier League and the Champions League with my new club."
Koscielny's arrival at the Emirates Stadium will increase the likelihood of experienced France defender William Gallas leaving the London club.
Arsenal have also signed Moroccan striker Marouane Chamakh from Girondins Bordeaux as they seek to improve on their third-place finish in the Premier League last season.
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