General Norbert Dabira, 73, was one of three senior figures who were jailed in 2018, the others being former presidential candidates General Jean-Marie Michel Mokoko and former minister Andre Okombi Salissa.
Dabira was released on Monday, one of his relatives said.
"The police came and dropped him off at home. There were lots of us there when he arrived, dressed in a suit and tie," the source said late Monday, asking not to be identified.
Dabira was once a close ally of 80-year-old Sassou Nguesso, who has been at the helm of the Republic of Congo, also called Congo-Brazzaville, for a total of 39 years.
He was previously inspector general of the armed forces and senior commissioner for reintegrating the armed forces, but fell out with Sassou Nguesso in 2018.
He was jailed on charges of endangering internal state security, accused of hiring "two elite snipers" to shoot down Sassou Nguesso's plane.
In separate trials that same year, Mokoko and Okombi Salissa were each handed 20-year jail terms, also for endangering state security.
The pair had run against Sassou Nguesso in bitterly disputed presidential elections in March 2016 that were followed by violence.
They rejected results that gave the incumbent a first-round victory with 60 percent of the vote, and called for a campaign of civil disobedience.
Before falling out with Sassou Nguesso, Dabira had been investigated in France in connection with the disappearance of 350 people who had returned to Congo-Brazzaville in 1999 from neighboring Democratic Republic of Congo, where they had taken shelter from a long-running civil war.