Saif al-Islam is to die by firing squad, the court ruled Tuesday. He gave evidence via video link at the start of the trial.
He is being held by a former rebel group from the town of Zintan that refuses to hand him over.
Those condemned to death were accused of war crimes and suppressing peaceful protests during the revolution that overthrew Mr. Gadaffi in 2011.
More than 30 close associates of the deposed leader were on trial.
Among them are the former head of intelligence for the Gaddafi regime, Abdullah al-Senussi, and former PM Baghdadi al-Mahmoudi. Both men were also sentenced to death.
Other defendants received sentences ranging from five years to life imprisonment, according to the BBC.
They faced charges including murder, kidnap and embezzlement, during the uprising that eventually toppled Col Gaddafi,” BBC says.
The trial opened in 2014, and has been criticised by human rights groups.
Saif al-Islam will be given the right to appeal the death sentence.
Photo Credit: REUTERS | The Times UK