The robbery last December was one of Spain's biggest jewel thefts ever, police said in a statement.
The robbers disarmed the jewellery store's alarms and broke into its secure chamber, stealing more than 1,700 luxury wristwatches.
Officers arrested the leader of the gang responsible for the heist along with "a well-known ram-raider" notorious for crashing vehicles through shop windows for robbery.
The six Chinese nationals detained were suspected of receiving the stolen watches and selling them on in China, the statement added.