Núñez, 29, arrives until the end of the season with Espanyol holding a voluntary option to buy, according to the club, alongside a separate obligatory purchase clause that would only be triggered if Espanyol qualify for Europe.
One of the main sticking points in negotiations was the defender's wages, among the highest at Celta Vigo; Espanyol have agreed to cover just over half of that figure, around 55%, while Celta will collect an initial loan fee reported elsewhere at €250,000.
The Basque centre-back becomes Espanyol's fourth signing of the summer, following the arrivals of Hartman, Álex Calatrava and Gabriel Moscardo, and gives head coach Manolo González fresh cover at the back after the exits of Fernando Calero, whose contract was not renewed, and Pablo Ramón, who moved to Racing de Santander, left the squad short of recognised central defenders alongside Miguel Rubio's uncertain future.
Núñez had also drawn interest from Osasuna, Rayo Vallecano and Valencia, where he began his senior career on loan, before Espanyol moved quickest to agree terms with Celta.
Núñez brings extensive experience in LaLiga, having represented Celta, Athletic Club and Valencia, as well as a spell in Serie A with Hellas Verona. He made his permanent move to Celta from Athletic in 2024 for a fee of around €7m, but fell out of Claudio Giráldez's plans following his return from that Verona loan and was left out of Celta's first two pre-season friendlies, confirming his exit was imminent. He is expected to be available for Espanyol's league opener against Levante on 16 August, with his formal presentation set for 12 August.
Speaking to the club's media, Núñez said the move had come together quickly because both sides wanted the same outcome, adding that he had always enjoyed playing at Espanyol's stadium and liked the project put to him. He described himself as a defender who competes hard in duels, positions himself well and tries to help his team build play from the back.