Rene Gonzalez "was released earlier today" from the Marianna federal prison, his attorney Philip Horowitz told AFP.
He did not provide details about where his client was headed, and said there will be "no further statements that I am aware of."
The 55-year-old Cuban must remain on US soil for three years as part of a "supervised release" programme, a requirement that has infuriated Havana, which considers Gonzalez a national hero.
The planned release has also angered US opponents of Cuba's communist government, like Republican lawmaker Ileana Ros-Lehtinen of Florida, who has called Gonzalez a "villain."
In February Gonzalez asked to be allowed to return to Cuba to be reunited with his wife and two daughters, but Judge Joan Lenard of the US District Court for Southern Florida rejected the request last month.
Horowitz said his client can serve out the three years anywhere in the United States, so he can leave Miami, a bastion of the Cuban exile community.
But earlier in the week he slammed the "supervised release" programme as punitive.
"Our contention is that it's three years of additional punishment away from his family," said Horowitz, who added that Gonzalez's wife was deported and is not allowed to return to the United States.
Miami-based newspaper El Nuevo Herald reported that Gonzalez was reunited with his two daughters and his father, who received US visas to be present when Gonzalez left prison.
Havana has acknowledged that the so-called "Cuban Five" were intelligence agents but said their aim was not to spy on the US government but solely to gather information on "terrorist" plots by Cuban expatriates in Florida.
Gonzalez, whose 15-year sentence was the lightest of the group, was arrested in 1998 with Gerardo Hernandez, Ramon Labanino, Antonio Guerrero and Fernando Gonzalez, and convicted on espionage charges in Miami in 2001.
The men argued they did not receive a fair trial in Miami in 2001 because of strong anti-Castro sentiments there.
On appeal, Labanino's life sentence was reduced to 30 years, Guerrero's life sentence was shortened to 22 years and Fernando Gonzalez's sentence was pared from 19 to 18 years.