Fifa handed the Blues the punishment after Lens made an complaint over the way Chelsea signed youngster Gael Kakuta in 2007.
The 18-year-old has also been banned from playing for the next four months.
A Fifa spokesperson stated: “This decision confirms our aim to ensure contractual stability. It is an important ruling for football in general.”
The full statement from Fifa reads: “On the occasion of its last meeting held on 27 August 2009, the Dispute Resolution Chamber (DRC) was called to pass a decision in a contractual dispute opposing the French club Lens to the French player Gael Kakuta and the English club Chelsea.
“The French club had lodged a claim with Fifa seeking compensation for breach of contract from the player and requesting also sporting sanctions to be imposed on the player and the English club for breach of contract and inducement to breach of contract respectively.
“The DRC found that the player had indeed breached a contract signed with the French club. Equally, the DRC deemed it to be established that the English club induced the player to such a breach.
“As a result the player was condemned to pay compensation in the amount of €780,000, for which the club, Chelsea, are jointly and severally liable, and sporting sanctions were imposed on both the player and Chelsea in accordance with art. 17 par. 3 and 4 of the Regulations on the Status and Transfer of Players.
“A restriction of four months on his eligibility to play in official matches has been imposed on Kakuta. Chelsea are banned from registering any new players, either nationally or internationally, for the two next entire and consecutive registration periods following the notification of the present decision. Furthermore, the club, Chelsea, have to pay Lens training compensation in the amount of €130,000.”