Ukrainian national Yevhen Panov pleaded not guilty to sabotage charges as a Russia-imposed court in annexed Crimea started his trial on April 16.
Panov called the charges against him "fantasies created by the investigators" and rejected them, Radio Free Europe / Radio Liberty said.
Russian authorities arrested Panov and another Ukrainian national, Andriy Zakhtey, in August 2016 and charged them with being a "saboteur group" that was planning a series of terrorist attacks on the peninsula.
Zakhtey, who pleaded guilty in an agreement with investigators, was tried in February and sentenced to 6 1/2 years in prison.
Kyiv has rejected Russian charges against the two men and has called their arrests "a provocation."
Russia has prosecuted and imprisoned several Ukrainians on what rights activists say are trumped-up, politically motivated charges since Moscow seized control of the Crimea region in March 2014.
In March 2017, the European Parliament called on Russia to free more than 30 Ukrainian citizens who were in prison or other conditions of restricted freedom in Russia, Crimea, and parts of eastern Ukraine that are controlled by Russia-backed separatists.