The Monastery of Saint John the Theologian, also known as the Cave of the Apocalypse, is a historic monastery located on the Greek island of Patmos in the Aegean Sea. The monastery was founded in the 11th century by Saint Christodulos and is considered one of the most important pilgrimage sites in the Greek Orthodox Church.

The monastery is built around the Cave of the Apocalypse, where Saint John the Evangelist is believed to have received the visions that inspired the Book of Revelation in the New Testament of the Bible. The cave is located beneath the main chapel of the monastery and can be visited by pilgrims.

The monastery itself is an impressive structure with fortified walls, towers, and gates, built on the site of an earlier Byzantine monastery. The interior of the monastery is richly decorated with frescoes, mosaics, and icons, many of which date back to the 12th century. The library of the monastery contains a collection of rare manuscripts and documents, some of which are over 1,000 years old.

The Monastery of Saint John the Theologian is also known for its spectacular views of the surrounding island and sea, as it is perched on a hilltop overlooking the port town of Skala. The monastery is open to visitors daily, but there are strict dress codes and photography is not allowed inside the monastery.