Muktar Robow served as al-Shabab's spokesman and deputy leader until he defected in August 2017.
"I disagreed with their creed, which does not serve Islamic religion," Robow said after defecting.
The United States had at one point offered a $5 million (€4.9 million) bounty for Robow's capture, a bounty which was removed at the Somali government's request. He was arrested in late 2018, shortly before he was scheduled to run for president of Somalia's southwestern Bay region.
The former al-Shabab militant had spent the last four years under house arrest after he fell out with ex-president Mohamed Abdullahi Mohamed, known as "Farmaajo."
Farmaajo's government accused Robow of organizing a militia in the southwestern Somali city of Baidoa. He was also accused of attempting to "undermine stability" in the country.