The Brazilian forward signed a new five-year deal with Barcelona in 2016, but the club was powerless to stop his move to France just a year later because the French club met the buyout clause in his contract.
In a statement, Barcelona said: “In this lawsuit, the club demands the amount already paid as a bonus for the renewal of his contract for breach of contract, €8.5m in damages, and an additional 10% in interests.
“Barcelona have taken these actions in defence of their interests, after Neymar’s contract was rescinded just a few months after he signed on until 2021.”
The club said it had lodged the demand against Neymar on 11 August with Barcelona’s employment court, eight days after his move to Ligue 1.
The Catalan side has also passed the claim on to FIFA and the French Football Federation to take what it sees as “the appropriate legal effects”.
Neymar’s family responded to the legal action. In a statement released by NN Consultoria, they said: “This news was received with surprise given that the player fully respected the contract valid at the time.
“As for the bonuses owed for the signature of the contract in 2016, stipulated in the contract and not paid by FC Barcelona… (Neymar) has already started formal proceedings to recover the money before the competent court.”
PSG added it was “surprised” by the move.
A statement said: “Paris Saint-Germain reiterates that, like Neymar Jr, it has always respected all applicable laws and rules in its dealings and, once again, regrets the attitude of FC Barcelona.”
It comes two days after Neymar criticised Barcelona’s under-fire board after scoring twice in his home debut for PSG.
The French side beat Toulouse 6-2 on Sunday.
Neymar told reporters: “There are people in charge that shouldn’t be there. Barca deserves much more and everyone knows that.”
Barcelona has also withheld a €26m bonus due to the striker for signing the renewal to his contract last year.
Josep Vives, a spokesman for the club, said on 4 August: “The player hasn’t fulfilled his contract with the club… given that breach the club will not pay this renewal bonus.”
PSG signed Neymar for a world record €222m (£203m) earlier this month.