He was found shot dead in his car in the capital, Tunis, prompting outrage.
The leftist politician had been a fierce critic of the then-ruling Islamist Ennahda party.
He claimed it turned a blind eye to violence perpetrated by extremists against secularists.
Although Tunisia hands out death sentences for the gravest crimes, no executions have gone ahead since 1990. Instead, they are usually commuted to life terms.
A total of 23 people have been charged in connection with Belaid’s killing.
“Justice has been done,” Prosecutor Aymen Chtiba said in response to the six sentences handed down on Wednesday morning.
The verdict was announced live on national television after 15 hours of deliberation, reports the AFP news agency.
Jihadists with allegiance to Islamic State claimed responsibility for Belaid’s assassination, as well as that of Mohamed Brahmi, another left-wing opposition figure, six months later.
These murders sparked mass demonstration from outraged Tunisians, two years after the pro-democracy Arab Spring uprising had begun in the country and spread elsewhere in the Maghreb and Middle East.
Tunisia is now governed by President Kaïs Saïed, who has himself been branded an autocrat after a series of power grabs including dissolving the country’s main legal body, sacking the prime minister and suspending parliament.