Ongi Monastery (Mongolian: Онгийн хийд, Ongiin Khiid) is the collective name for the ruins of two monasteries that face each other across the Ongi River in Saikhan-Ovoo district of Dundgovi Province, in south-central Mongolia.

The older southern complex consisted of various administrative buildings as well as 11 temples.

Founded in 1660, it was one of the largest monasteries in Mongolia and housed over 1000 monks at its height.

Both complexes of Ongi Monastery were completely destroyed in 1939 during anti-religious purges carried out under Khorloogiin Choibalsan, the leader of the Communist Party of Mongolia.

A large number of ruins including a tall stupa can be seen on the river and on the surrounding hills.