The court said it had found Hassan Hanafi Hagi guilty of involvement in the killings of at least five journalists between 2007 and 2010.
Hagi had joined the Islamist group in 2007 and worked for its radio station Andalus.
He was accused of threatening journalists who reported critically about al-Shabaab and of helping the Islamists target them.
“It is a historic day for the media community in Somalia today,” journalist Burhan Diini said about the trial in Mogadishu.
Hagi was arrested in Kenya in 2014. He is due to be executed by a firing squad.
Global media watchdogs consider Somalia one of the most dangerous countries to work as a journalist. More than 60 journalists have been killed in the Horn of Africa country since 1992, according to the Committee to Protect Journalists.
Source: GNA
