"I am signing for the Champions League winner," Hazard, 21, said on his Twitter feed.
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 20 goals this season, including a hat-trick in his farewell game, as they finished third to qualify for next season's Champions League play-off.
He announced in October that he would leave Lille at the end of the season.
Hazard made his league debut for Lille as a 16-year-old and quickly had scouts flocking to the club's Metropole stadium. His performances earned the Ligue 1's top prospect award for successive seasons, before he secured the full accolade in the last two campaigns.
With tremendous pace and an eye for goal, Hazard prefers to play on the left flank and his penetrating runs and tight ball control are a menace to any defence.
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.
