The Apostolic Prefecture of Ulaanbaatar (Latin: Praefectura Apostolica Ulaanbaatarensis) is a Roman Catholic Latin apostolic prefecture (pre-diocesan missionary jurisdiction) located in Mongolia, with its territory consisting of the entire country.

Its cathedral episcopal see is the Cathedral of Saints Peter and Paul, in the capital city of Mongolia, Ulaanbaatar.

It is exempt, i.e. directly subject to the Holy See, not part of any ecclesiastical province.

On 28 August 2016, the Apostolic Prefecture of Ulaanbaatar celebrated the ordination of the first native Mongolian priest by Msgr.

As of 2014, the prefecture had 919 Catholics (0.03% of 3,227,000 total) on 1,564,120 km2 in six parishes and a mission with 17 priests (14 religious and three diocesan), 61 in religious life (43 female, 18 male) and 2 seminarians.