Saint Achillius Basilica, Prespa, Byzantine basilica on Agios Achillios island, Greece.

Agios Achillios Basilica is a Byzantine church on an island in Lake Mikri Prespa, built with three aisles, a narthex, and vaulted side chambers. Remnants of the original structure survive in the eastern section, including traces of wall paintings from different periods.

The basilica was built between 986 and 1002, according to Nikolaos Moutsopoulos, likely under Tsar Samuel, who moved his capital to Prespa in 986 and brought the relics of Saint Achillios from Larissa. It served as the seat of the Bulgarian Patriarchate around 997. It was damaged by Normans in 1041 and destroyed again in 1073, later rebuilt by Archbishop Theophrastos of Ohrid.

The basilica served as a major pilgrimage site centered on Saint Achillios, whose remains drew worshippers seeking spiritual connection to the saint. The cult surrounding him shaped the island's role as a sacred destination for centuries.

Access to the island is via a wooden bridge from the mainland, serving as the main pathway to the archaeological site. The island sits in shallow water, making visits easier in good weather but potentially difficult during bad conditions or high water levels.

Wall paintings from two distinct historical periods were removed from inside the basilica and are now displayed at the Archaeological Museum of Florina. These fragments provide glimpses into the artistic phases the building underwent over time.