Yonatan had written on Facebook that the government had used "force against the people instead of using peaceful discussion with the public."
Yonatan Tesfaye was arrested in December 2015 after writing on the social media platform that the government had used "force against the people instead of using peaceful discussion with the public."
While Yonatan's lawyer and defence witnesses argued the former spokesman of the opposition Blue Party was exercising his right to free speech, judge Belayhun Awol ruled the comments "exceeded freedom of expression" and amounted to encouraging terrorism.
"I think the government's intent and what it seeks is this: to restrict others from speaking freely," Yonatan's lawyer Shebru Belete Birru told AFP after the verdict.
The guilty verdict for "encouragement of terrorism" means Yonatan faces a possible sentence of between 10 and 20 years under the country's anti-terrorism laws, which have been criticised by rights groups and Ethiopia's allies, such as the United States, for being used to stifle dissent.
Yonatan's comments came shortly after protesters belonging to the country's largest ethnic group, the Oromos, took to the streets in towns outside the capital Addis Ababa, claiming a plan to expand the city's boundaries into their region amounted to a land-grab.
The protests led Ethiopia's government to declare a state of emergency last October, which was extended for another four months in March.
Yonatan was originally charged with being a member of the banned separatist Oromo Liberation Front, but prosecutors changed the charges against him last year.
Shebru said he plans to appeal the verdict.