Tiny St Mary's Island lies on Veliko Jezero, not far from its southern shore. Boats (included in the park admission price) head here at least hourly during park opening hours from Mali Most, the bridge near the channel between the two lakes, and from Pristanište. The island's Benedictine monastery was founded in 1198 but has been rebuilt several times, adding Renaissance and baroque features to the Romanesque structure.
The monastery was closed in 1809 following the Napoleonic conquest and housed state offices up until 1960. It was then converted into a hotel, which closed in 1991 during the war. It has since been returned to the Catholic Church, which is in the process of restoring it. The complex includes a large church, a couple of tiny chapels and the old monks' quarters, which now has a restaurant in its basement. There are also ruins of a Roman building in the island's centre and stables with a donkey.
The original monks were responsible for deepening and widening the passage between the two lakes, building a tidal mill to take advantage of the rush of sea water.