The deal includes an option to extend the loan for a second season or make the Spaniard's move permanent, LaLiga side Bilbao said.
The 33-year-old returns to his former club where from 2011-2014 he made nearly 130 appearances in all competitions for the Basque side.
In his first season, he helped Bilbao reach the finals of the Copa del Rey and the Europa League, where they lost 3-0 to Atletico Madrid.
?️ @AnderHerrera's first comments upon returning to #AthleticClub ?
— Athletic Club (@Athletic_en) August 27, 2022
?️ "Coming back here makes me so happy. It's very exciting to be able to continue this journey."
✊ "It's an amazing challenge. I can't wait."#OngiEtorriAnderHerrera ?⚪️ pic.twitter.com/ExnyEmnlcV
"It is a great joy to be back," Herrera told the club's Twitter feed.
"It's been eight years since I left but I was very happy here, those were three great years.
"Athletic hasn't stopped growing since I left. The challenges are great, the demands are great because the quality of the team demands it."
Herrera left Bilbao for Manchester United ahead of the 2014-15 season and during a five-year spell in England, he lifted the FA Cup, League Cup and Europa League trophies.
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