The former France international moved to Wigan from Birmingham in 2008, but has played just 20 Barclays Premier League games for the Latics.
"I want to fight as I did at Wigan," said Kapo. "I can't wait to sense the smell of the dressing room and above all the smell of victory with Boulogne.
"I have the fortune that a top-flight team put out their hand to me and I seized it.
"Not knowing too much about the club, I found out about it by asking other players.
"They all only told me good things about it. So didn't hesitate long before signing. I really hope we are going to stay up. It's feasible. In any case, I'm really happy to be here."