The 65-year-old spent almost two decades spying for Cuba while employed as an analyst at the Defence Intelligence Agency.
After her arrest in 2001, officials said she had almost entirely exposed US intelligence operations on the island.
One official said she was among "the most damaging spies" caught by the US.
After her arrest, Montes was accused of supplying the identities of four US spies and oceans of classified material. She was handed a 25-year prison sentence, with the sentencing judge accusing her of putting the "nation as a whole" at risk.
Working secretly using a false appearance in order to get information for the police or government.
• TO SENTENCE
To decide and say officially what a punishment will be.
A place, especially including buildings, where a particular activity happens.
Famous for something bad.
Source: The Independent / BBC News