"I am signing for the Champions League winner," Hazard, 21, said on his Twitter feed (@hazardeden10).
Hazard, named the best Ligue 1 player in the last two seasons, had been expected to join either Premier League champions Manchester City or runners-up Manchester United after he said he would choose between the two.
The gifted playmaker, capped 27 times by Belgium, led Lille to the French title in 2011 and scored 17 goals this season as they finished third to qualify for next season's Champions League playoff.
He announced in October that he would leave Lille at the end of the season.
Chelsea won the Champions League for the first time this month, adding to their FA Cup final success. The London side endured a disappointing league campaign, however, finishing sixth after sacking manager Andre Villas-Boas in March.
Former player Roberto Di Matteo took interim charge and guided the team to an unexpected Champions League final win over Bayern Munich.
