The 21-year-old was the subject of interest from several big clubs - among them Manchester City, Manchester United and Tottenham - but opted to join the European champions instead.
But who is he? How did he come to prominence? And why have the Premier League's top sides been throwing money - a reported £35m transfer fee alone - at him to try to secure his services?
We run down everything you need to know about Chelsea's new signing.
A 21-year-old Belgian midfielder who has been at Lille since he was 16. He can play as a left-sided winger, but prefers to sit in the number 10 role, just behind a lone striker, both scoring and creating goals.
Blessed with prodigious speed, technique and killer instinct in front of goal, he has been widely hailed as the new Lionel Messi. His 20 goals and 15 assists last season suggest that he is already on his way to being one of the best players in the world, and his preference for a playmaker role does lend him to that Messi comparison.
The fact that he apparently demanded to be able to announce the move himself on his own Twitter page suggests not. As does his reported insistence that his future team guarantee him a first team place in his preferred central midfield role. And he wasn't shy about criticising the Argentine last year either, saying that while Messi is "the best player around at the moment" he "does not have the elegance that Zidane and Platini had".
Put it this way: when he was subbed off during a recent Euro 2012 qualifier he stormed out of the stadium to go for a hamburger with his family rather than stay to watch the last half hour of the match. The petulance cost him a three-match suspension from coach Georges Leekens, though he is such a key man for the side that his ban was later cut short. Still, Joe Cole has praised his arrogance, likening him to Wayne Rooney with his fierce competitive streak.
No official fee announcement has been disclosed - and indeed neither Lille nor Chelsea have officially confirmed the deal - but it is thought to be just over £35 million. In addition to that the player has insisted on £100,000 a week after tax, which will cost Chelsea £9m a year, while Hazard's agent John Bico has apparently negotiated a £6m fee for himself for brokering the deal. The Daily Telegraph estimates that the grand total for all this will come to £78m.
Only time will tell. His talent is not in doubt, though many have pointed out that being one of the best players in Ligue 1 is a very different thing to being one of the best players in La Liga, Serie A or the Premier League. Yet his starring role for Belgium's emerging national side - who play England this weekend in a pre-Euro 2012 friendly - suggest that he can mix it at any level. Either way the jury is still out, with Manchester City reportedly drawing back from their efforts to sign him over fears that he wasn't worth quite as much as he thought.
Belgium team-mate
"Hazard is a great talent. The best in the French championship. With Lille, he played sometimes on the left side, and more often just behind the forward.
"He is a very fast player, skilful with the ball. He plays with both feet. He's also very tough: at Lille, he has played over 100 consecutive games in the league. This season, he has become more efficient. As proof, he scored 20 goals and gave 15 assists. He has not many faults. But if I had to find one, I'd say that his heading ability could be improved.
"I think he has all the qualities to become a star in England. Premier League defences will suffer. At Chelsea, the team will probably be built around him. The future coach should install him just behind Torres: he would have so much freedom and would hit the ground running."
