A Barcelona court ruled in the La Liga club’s favour on Friday, ordering Neymar to pay €6.7m (£6m) to Barca, while dismissing his claim.
The Paris Saint-German and Brazil forward had claimed he was owed €43.6million by Barça in unpaid bonuses.
Neymar insisted he was due the huge sum as a loyalty bonus, having signed a new five-year contract with the Catalan giants in 2016.
The Brazil international, who joined PSG for a world-record fee of €222m in August 2017, has instead been told he must fork out to his former employers – although an appeal could be lodged.
Barca, who has been linked with a move to re-sign Neymar, welcomed the verdict on Friday.
A club statement said: “FC Barcelona expresses its satisfaction with the verdict announced today by Social Court 15 in Barcelona in relation to the lawsuit involving FC Barcelona and the player Neymar Jr regarding the amount of the signing bonus in the final renewal of the player’s contract.
“The ruling has fully dismissed the player’s claim for payment of 43.6 million euros and has accepted a large part of the defence presented by FC Barcelona, as a result of which the player must return €6.7million to the club.
“Since the player’s representative is entitled to appeal this decision, the club shall continue to fervently defend its legitimate interests.”