The 27-year-old's loan at Elche has concluded, and a permanent transfer to Greece is now being finalised, according to Jijantes and confirmed by MARCA.
Pena joined Barcelona's academy from Villarreal in 2012 and progressed steadily through the youth ranks, showing strong reflexes, positioning and composure along the way.
Ronald Koeman promoted him to the first team in the 2021-22 season, but with limited opportunities, he was loaned to Galatasaray in the winter window under Xavi Hernandez, making eight appearances in Turkey.
His Barcelona debut came on 1 November 2022 against Viktoria Plzen in the Champions League, but he struggled for regular involvement in the seasons that followed, with Marc-Andre ter Stegen, Wojciech Szczesny and Joan Garcia among the goalkeepers limiting his path into the side. His most extended run of first-team football came during spells when Ter Stegen was sidelined by injury.
This season he returned to action on loan at Elche under Eder Sarabia, making 16 appearances for the newly-promoted club.
Pena, who has a contract with Barcelona until June 2029, will move to Panathinaikos for a fee in the region of €3m, with negotiations well advanced and all parties expecting the deal to be finalised next week.
In a Barcelona shirt, Pena made 45 appearances across all competitions, conceding 64 goals and keeping 10 clean sheets.
His departure is not the only goalkeeping situation Barcelona sporting director Deco is working through this summer. Ter Stegen is also expected to leave the club in the coming weeks, with the futures of several other goalkeepers in the squad also still to be resolved.
Pena's struggle to establish himself at Barcelona reflects the sheer depth of competition the club has carried in goal over the past four years, with a succession of established names making it almost impossible for a homegrown product to force his way into a settled role, regardless of his individual quality.
Head coach Hansi Flick has previously acknowledged that the situation has not been ideal for the goalkeeper, but stressed that decisions ultimately have to be made in the best interests of the squad as a whole, leaving Pena to seek regular football elsewhere after more than a decade at the club.