According to the Spanish newspaper El Mundo Deportivo, the club did not make the right tax deduction when paying the Brazilian's salary during his time at the club.
The newspaper has read a report from the Spanish tax authorities.
The tax authorities believe that Neymar has benefited from "unfair wealth".
According to information, Barcelona will now try to get the money back, and go to court.
According to the newspaper, Neymar owes the Spanish state 34.6 million euros, about 350 million kronor, in tax debts.
Several football stars have previously been convicted of tax crimes in Spain, including Lionel Messi, Cristiano Ronaldo and coach José Mourinho.