The Monastery Grigoriou is an Orthodox Christian monastery in the monastic state of Mount Athos in Greece. The church is dedicated to Agios Nikolaos (St. Nicholas), thus the monastery celebrates on the 6th of December. According to a handwritten codex, it was founded by a Serbian monk, Saint Grigorios Sinaitis in 1345. It is built on a rock 30 meters high, on the southeastern side of the peninsula, an hour away from the neighboring Monasteries of Simonopetras and Dionysiou.
Little is known about the founder himself, other than he came to Mount Athos to pursue asceticism. Before the construction of the monastery, he dwelled in a cave, which still exists, about 20 minutes up from the Monastery.
The monastery was burnt down twice, first in 1496 and later in 1761. It recovered from the destruction of 1500 and was re-established. The big fire of 1761 caused great damage, with almost all the relics and documents to have been destroyed. After the second fire, reconstruction began almost immediately. Additional damage was suffered during the Greek revolution against the Turks in 1821. Restoration work has been accomplished over the years, supported by various benefactors including the princess of Moldavia, Phanariotes and archbishops of Hungro-Wallachia.
The katholikon which is dedicated to St. Nicholas, was built in 1768. Its walls were decorated with frescoes, painted in 1779 by the monks Gabriel and Gregory from Kastoria. The frescoes present scenes from the Old Testement. A narthex was added to the katholikon in 1846. Between the Katholikon and the narthex there is the chapel of Saint Grigorios, dating from 1851. The monastery owns 10 chapels of the 18th and 19th century, of which the more distinguished are St. Gregory's and St. Anastasia of Rome. It also features 4 cells in Κaryes.
Its library holds 297 manuscripts, many other documents, and some 6,000 printed books, including the unique manuscript of Shepherd of Hermas. This manuscript is an illuminated thirteenth-century Gospel Book. The monastery’s treasury is very rich in relics from various eras. Among the Monastery's treasures are a small part of the True Cross, relics of saints, sacred vessels and vestments.