The Washington state product, who was a backup keeper for the USA at the 2006 World Cup, played for the USL's Seattle Sounders and MLS's Colorado Rapids before going to England to join Fulham in 1999.
Hahnemann played only twice for Fulham before parlaying a loan to Reading into a full-time contract in 2002. Hahnemann was a fan favorite at Reading and helped it win promotion to the EPL in 2007, but the Royals went back down after one season.
"Marcus has been a tremendously consistent and reliable goalkeeper for many years, and has without doubt been one of our most important players in the last decade," Reading's director of football Nick Hammond told the Club's official website.
Hahnemann said he was disappointed that his contract wasn't renewed and said he hopes to play another year in England.
"I am a little bit shocked, but when your contract is allowed to run out, you kind of know," he told the Evening Post. "It is so disappointing. I thought I was going to finish my career at Reading and now I'm not. That is the hardest part.
"I love it here. All our friends are here and the family are settled. ... I still want to carry on playing and I want to stay in England."