A deal was agreed last week but the player has now signed the contract taking him to the Emirates Stadium for an undisclosed fee.
"It's a great feeling to be an Arsenal player," he said. "I'm very happy. I've always dreamed to play for this club. I've achieved this dream now and I am so happy. It's one of the most beautiful days of my life."
The 24-year-old scored 18 goals in 49 games last season, helping Lille to a league and cup double.
Manager Arsene Wenger was keen to praise the striker's versatility: "He can play on the right, on the left and through the middle.
"He has qualities that I find very important, like his movement off the ball. I think he is a very good addition to the squad."
The Ivorian has also won 27 caps for his country, including appearances at the 2008 Beijing Olympics and 2010 World Cup.