The 31-year-old defender signs a three-year contract after a delayed transfer from Al-Nassr was eventually approved by FIFA.
The move was initially blocked when Al-Nassr failed to submit the required documents before the September 1 transfer deadline. However, following an appeal from the Spanish Football Federation (RFEF), FIFA granted an exception and allowed the registration.
"Aymeric Laporte returns home," the club said. "He signs until June 30, 2028."
Laporte, who made 222 appearances for Athletic between 2012 and 2018, re-joins under manager Ernesto Valverde, having previously played under him during his first spell. He went on to win five Premier League titles and the Champions League with Manchester City before a brief stint in Saudi Arabia.
"I'm very happy because this is everything I wanted," Laporte said. "I'm really looking forward to helping the team and stepping foot in San Mames."
His return comes at a critical time, with Yeray Alvarez suspended and Unai Egiluz injured, leaving Athletic thin in defense.
Laporte's transfer also comes amid wider debate in European football over the globalization of domestic matches. UEFA recently postponed decisions on staging La Liga and Serie A matches abroad, amid pushback from fan groups and political leaders.
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