Koutloumousiou monastery (Ελληνικά. Μονή Κουτλουμουσίου) is an Eastern Orthodox monastery at the monastic state of Mount Athos in Greece. The monastery ranks sixth in the hierarchy of the Athonite monasteries.
The monastery was raised with the help of voivodes Nicolae Alexandru and Vladislav Vlaicu from Wallachia.
Among the treasures of the monastery is the purported largest relic of the True Cross.
The library of monastery contains 662 manuscripts and approximately 3,500 printed books.
It has 20 working monks.