Destructive large earthquakes occur not only along major plate boundaries but also within the interior of plates. To establish appropriate safety measures, identifying intraplate active faults and the potential magnitude of associated earthquakes is essential before an earthquake occurs. This study was conducted to document the geomorphic expression of a previously unrecognized 50‐km‐long active fault in Ulaanbaatar, the capital of Mongolia. Mapping of the fault was accomplished using the Advanced Land Observation Satellite elevation dataset provided by Japan Aerospace Exploration Agency (JAXA), a stereo‐scope interpretation of CORONA satellite images, the emplacement of trenches across the fault trace, and field study. The Ulaanbaatar fault (UBF) is marked by fault scarps on the surface and left‐lateral stream deflections. The fault displaces late Pleistocene deposits and is thus considered to be active. Based on the length of the fault, the UBF is believed to be capable of causing earthquakes with magnitudes greater than M 7 and subsequent associated damage to buildings and heavy causalities within the metropolitan area. We strongly suggest that building resistance requirements in Ulaanbaatar should be revised to mitigate for the potential of extensive seismic damage. The results of this study can be used to revise the seismic hazard map and stipulate a new disaster prevention strategy to improve public safety in Ulaanbaatar. It is also possible that there may be other active faults in the vicinity of Ulaanbaatar, and these require investigation.