"The evidence collected in the case proved that he had not only been given the assignment, but had also performed specific spying actions," Gintare Daugelaite, spokeswoman for the court, told BNS.
The Prosecutor General's Office sought a prison sentence of eight years for the Klaipeda resident. Under Lithuanian laws, spying is punishable by three to 15 years' imprisonment.
The ruling is not final and can be appealed to the Lithuanian Court of Appeals.
Sesel did not admit to any of the charges brought against him.
According to the Prosecutor General's Office, the man, who was born in Russia in 1974 and lived there for some time, was recruited by a Russian intelligence organization and carried out certain tasks related to the collection and transfer of information to that organization.
Prosecutors say they gathered evidence showing that from 2015 until his detention in December 2017, the defendant collected information on the Lithuanian Navy, NATO military ships, and facilities in the city and port of Klaipeda, including those of strategic importance to national security.
Five people have already been convicted of spying in Lithuania since 2014. Some of them have already served their prison terms.