"Lille confirm the signing of Salomon Kalou, who passed a medical this morning after arriving at the club last night," the 2011 French champions announced on their website.
"Agreed in principle at the end of June, the European champion has signed a four-year contract until June 2016."
The player, 26, explained his decision to join the club near the Belgian border.
"Lille's project, notably with the impending new stadium, is ambitious and really pleased me. Given what I already knew about the club, there is nothing more you could ask from Europe's biggest clubs.
"I am excited about coming here and look forward to meeting the warmest of fans."
Kalou, who scored 60 goals in six seasons for Chelsea, was a free agent after leaving the Champions League winners when his contract expired at the end of the season.
He is a direct replacement for Eden Hazard, who himself moved to Stamford Bridge at the start of the summer.
Lille - who finished third in Ligue 1 and will play in the Champions League qualifiers - also lost on-loan Liverpool playmaker Joe Cole, who is expected to resume his career at Anfield, but signed France midfielder Marvin Martin in June.