Griezmann moved to Camp Nou in July after a lengthy transfer saga and some debate over the France forward's release clause, which dropped from €200million to €120m at the end of June.
Atletico were less than happy with their rivals' conduct, claiming they reached an agreement with the player when the €200m fee was still active, and raised the issue with the Royal Spanish Football Federation.
The governing body returned a ruling on Thursday, finding Barca guilty of violating article 126 of its disciplinary code.
Barça fined €300 (yes, really, we haven't missed out any zeros) for starting talks with Antoine Griezmann while he was under contract with Atleti, without notifying them first. No punishment for Griezmann himself. pic.twitter.com/wpo01Fn4zo— The Spanish Football Podcast (@tsf_podcast) September 26, 2019
A symbolic monetary penalty was imposed, while Griezmann escaped without sanction.
The decision can be challenged if an appeal is lodged within the next 10 business days.
It had been reported Barca were facing a one-game stadium closure, but no such punishment was handed down.