The club appealed and were cleared to sign new players in January's transfer window pending a final decision by CAS which was handed down on Thursday when Chelsea were cleared.
“The Court of Arbitration for Sport has issued an award today ratifying the agreement reached by Chelsea FC, Racing Club de Lens and the French football player Gael Kakuta,” CAS said.
“The CAS has noted that, in the agreement, the two clubs and the player have recognised that the contract between the player and RC Lens was not valid.