The 21-year-old is considered one of Europe's best young players, and he has already been given an emotional send-off from his current club Lille, with whom he won the Ligue 1 title in 2010-11.
And, after teasing fans for several days and saying he would reveal all on Monday afternoon, he finally made his decision, saying in French: "Something promised, something due - and I will join the champions of Europe."
He confirmed his decision in English in a subsequent Tweet.
Hazard had been mulling over offers from Manchester City, Manchester United and the Blues, with the West London side emerging as favourites after he reacted positively to their Champions League victory.
Before Chelsea's win Hazard had said he was interested in moving to one of the two Manchester clubs.
He is expected to cost around £25 million, plus wages of £170,000-a-week.
Hazard had said he would choose the team that guarantees him the most playing time in his favoured 'number 10' position.
He said last week: "I scored 20 league goals also had the most assists in the league (16), and I never believed I could possibly come up with such a fantastic set of statistics.
"Certainly I could not have made such a decisive contribution had the coach at Lille not changed my position.
"It is all down to that. I have always dreamed of playing the No 10 role, and wearing that number on my back, and you can see what a difference it makes from how many goals I have scored and set up for others. That will be a consideration when I decide on my club. Where I will play will influence my thinking on who I should join."
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