United Nations faced up to 10 years in jail after a court in Addis Ababa found him guilty on Monday of "participating in a terrorist organisation".
"The defendant has not convinced us that he did not commit a crime... he's guilty," Judge Mulugeta Kidane said.
Abdurahman Sheikh Hassan, who was based in Ethiopia's troubled south-eastern Ogaden region, was charged last July with having links to the Ogaden National Liberation Front (ONLF), a secessionist rebel group.
He is charged along with Sherif Baido, whom the charge sheet lists as a senior member of the ONLF and was convicted on the same charge in absentia, according to Kidane.
The two men face five to ten years in prison. The court is expected to deliver a sentence on Friday.
Hassan was arrested after negotiating the release of two UN World Food Programme (WFP) officers kidnapped in the Ogaden.
Judge Mulugeta said defence witnesses failed to prove Hassan did not have links to ONLF members.
The key evidence used by the prosecution was recordings of phone conversations between Hassan and his co-defendant.
The judge said the voices of Hassan and Baido had been matched to those on the tape after it was analysed by the cybercrimes unit.
"It has been proved by the cybercrimes section that it was their voices," he said.
Hassan - an ethnic Somali who does not speak Ethiopia's main language, Amharic - stood expressionless as the judge read the verdict in Amharic. He appeared in court wearing a clean white shirt and holding Islamic prayer beads.
The prosecution called for the court to impose a strong sentence against Hassan, saying his case should serve as a warning to would-be offenders.
"We want the court to take this as a really serious offence, he has a connection to the ONLF and he is a real danger to the country," the prosecutor told the court.
But Hassan's lawyer Netsanet Getahun asked the court to be lenient towards her client.
"My client hasn't harmed anyone, he hasn't done any concrete damage... we want the court to be considerate because he has already served one year in jail," she said, adding that Hassan suffers from high blood pressure and heart disease and has a large family to support.
Rights groups have criticized Ethiopia's 2009 "anti-terrorism" legislation for being far-reaching and used to stifle peaceful dissent and freedom of expression. Close to 200 people, including opposition members and journalists, were charged under the law in 2011.
In December, two Swedish journalists were sentenced to 11 years in prison after an Ethiopian court found them guilty of supporting ONLF rebels in the Ogaden.
Hassan was arrested in the remote Ogaden region, where ONLF militants have been fighting for independence since 1984, claiming they have been marginalised by Addis Ababa.
Oil and gas reserves in the region have brought hopes of wealth but also fears of increased conflict.