Meanwhile, Moscow claims the sailors violated its maritime border near the peninsula which it seized from Ukraine in 2014.
Ukraine issued an appeal to ITLOS, asking Russia's release of the detained ships and crew members in mid-April.
Although ITLOS sided with Ukraine in the dispute, Russia refuses to recognise the jurisdiction of the body.
The Ukrainian vessels had tried to pass through the Kerch Strait, the only access to Ukrainian ports on the Sea of Azov.
But Russia has controlled the Strait since annexing Crimea, and its Coast Guard boats fired on the vessels before boarding them, BBC reports.
Russia has held the sailors in Moscow ever since. They may face up to six years in jail if found guilty.
The Russian Federation must proceed immediately to release the Ukrainian soldiers and allow them to return to the Ukraine,
...said the tribunal's Judge Jin-Hyuan Paik in a ruling.