The 12-judge panel agreed to hear a claim filed by Ingrid Sartiau, who alleges she was born in 1966 as a result of a relationship between her mother and the king, who was the crown prince at the time and already married.
Sartiau has asked the court to order a paternity test.
The former king, who has faced numerous allegations of infidelity, was given 20 days to respond to the lawsuit.
Previous claims be other individuals have been rejected. In order for a paternity claim be accepted, the plaintiff is required to submit documentation of the claimed relationship.
Sartiau's lawyer, Jaume Paralos, told CNN that his client's mother has agreed to testify if a trial is convened. But, he said, "we would like to reach settlement. My client never wanted it to get to this point."
Juan Carlos abdicated the throne in June, passing the crown to his son, Felipe VI.