The Spaniard, whose new deal will run until 2017, has also claimed that he is "really looking to" representing the club in the Champions League next season.
"Hello to everyone! I wanted to convey my happiness at renewing my contract with Athletic Club. For me it's a pleasure and an honour to defend this shirt and this club crest," he told Bilbao's official television channel.
"I'm really looking forward to getting this exciting new season started and to prepare for the qualifying game in the Champions League, which is really important for all of us.
"I want to send a big hug to all the fans, the club and all my teammates and all the people who form a part of this great family. See you soon!"
Muniain has been with Bilbao since 2005.