The French club, who sold Eden Hazard to Chelsea in June, have snapped up the free agent after his contract at Stamford Bridge ended at the end of last season
The Cote d'Ivoire international has been linked with a host of clubs after being released by Chelsea at the end of the season.
However, it soon emerged that he would be moving to Lille, with the deal picking up pace after the news that the 26-year-old arrived in the Flanders city on Friday evening for a medical.
"Lille formalises the arrival of Salomon Kalou who this morning underwent the traditional medical procedure after his arrival at Domaine de Luchin yesterday," a club statement read.
"Released late June, the contract of the 26-year-old European champion is for four seasons. The former Chelsea player and has committed until 2016.
"The arrival of a European champion, Salomon Kalou, to Lille is certainly among the highlights of the 2012-2013 transfer window.
"Ambitious to enter a new stadium, the club and [club president] Michel Seydoux is pleased to have attracted a player of this calibre in their squad."
Kalou spent six seasons in West London, making more than 240 appearances for Chelsea, scoring 60 times, but found playing time increasingly rare this season, turning out only 12 times in the Premier League.
The Ivorian is Rudi Garcia's fourth summer signing at the club, joining Marvin Martin, Steeve Elana and Viktor Klonaridis at Les Dogues.