Champions League contenders Chelsea were cleared to make signings in the January transfer window on Friday, when the Court of Arbitration for Sport (CAS) halved a transfer ban imposed on the club last February.
CAS said in a statement it had also halved Chelsea's fine to 300,000 Swiss francs.
The West London club, last season's Europa League champions, had been handed a two window ban by world football body FIFA for breaching rules on the international transfer and registration of players under 18.
They served one window in the last close season.
"(Chelsea) is banned from registering any new players, either nationally or internationally, for one entire registration period, which the club already served during the 2019 summer registration period," CAS said.
The club, currently fourth in the Premier League, had filed an appeal with CAS in June.
After a lengthy investigation into the registration of players at academy level, FIFA had declared Chelsea in breach of article 19 of the regulations in the case of 29 minor players.
FIFA said the club had also breached an article in connection with agreements it concluded concerning minors and which allowed it to influence other clubs in transfer matters.
CAS said the breach involved "a significantly smaller number of players".
"The Sole Arbitrator found that CFC (Chelsea) did violate Articles 19.1 (related to the international transfer of minors) and 19.3 (related to the first registration of minors) of the RSTP," the statement said.
It added, however, that the arbitrator found only "about a third of the violations" declared by FIFA.
"In addition, the violations of other RSTP (Regulations on the Status and Transfer of Players) rules were found to be less serious than those attributed to Chelsea FC by FIFA," the statement added.