The Monastery of St. John the Theologian on Patmos, is possibly the most important monastery of the Aegean Sea. It was founded by the Blessed Christodoulos (1088) who was granted the island of Patmos exempt from taxation by an imperial chrysobull of Alexius I Comnenus.

Built on a mound, on the site formerly occupied first by a temple dedicated to the goddess Artemis (Diana) and then by an Early Christian basilica, the Monastery is surrounded by an irregular fortified quadrangle whose successive building phases span the period from the late 11th century to the 17th century.

The conventual church belongs to the tetrastyle cross-inscribed domed architectural type. The present aspect of the compound is the result of several successive additions, alterations and modifications. The conventual church, refectory, cells and εστία date from Byzantine times. Auxiliary structures are more difficult to date.

The murals of the conventual church date from the early 17th century, while those in the chapel of the Virgin, south of the conventual church, date to the late 12th century and are contemporary with the frescoes of the refectory, to the northeast of the main church. The cells are ranged along the south side of the defensive enclosure. There are also post-Byzantine chapels dedicated to St. Basil, St. Nicholas, the Holy Cross, St. John Prodromos and the Holy Apostles (1603). Two smaller ones stand outside the enclosure: St. George and St. Onuphrius (1611).

The Library and Treasury of the Monastery are both exceptional; some of the rare artifacts in its collections may be admired in the Monastic Museum.