The Gunners, making their first venture into the transfer market this summer, have moved for the highly-rated French youngster, who scored 11 goals in 24 appearances with the Ligue 1 side.
Sanogo was out of contract, however, and joined as a free agent.
Arsène Wenger said: “Sanogo is a good young signing for us. He has shown that he has potential with his recent performances for Auxerre and also for the France Under-20 side. We are looking forward to Yaya joining us and continuing his promising development.”
Sanogo will join up with his new team-mates at the Emirates after France finish their involvement at the Under-20 World Cup in Turkey. Sanogo has scored twice in three games as France reached the knock-out stages.