Athletic Club had moved to bring Laporte back to San Mames on September 1st, but the transfer did not go through before the deadline, with Spanish media reporting Al Nassr had not filed the details into FIFA's Transfer Matching System.
Earlier this month, FIFA rejected a request from Spain's football federation (RFEF) to make an exception for Laporte.
Al Nassr did not immediately respond to a request for comment.
Athletic Club said in a statement on Thursday that it had authorised RFEF to seek an International Transfer Certificate from its Saudi counterpart and later confirmed the move had been completed, with the 31-year-old signing a three-year contract.
Laporte clocked up 222 appearances for the Basque club before joining Manchester City in 2018, winning five Premier League titles, two FA Cups, three League Cups and the Champions League, before moving to Al Nassr in 2023.
Athletic Club are second in LaLiga with three wins from three matches and next host Alaves on Saturday.