All nine are Yemeni but have family ties to Saudi Arabia.
None of the men had been charged and all but one had been cleared for release from the US base in Cuba since at least 2010. One was approved for release by a review board last year.
They could not be sent to their homeland because of instability there.
The prisoners include a frequent hunger striker whose weight had dropped to as low as 34 kg at one point.
The release announced today in a Pentagon statement brings the Guantanamo prisoner population to 80, including 26 cleared men expected to leave by the end of the summer.