Rosebud County confirmed its first case of COVID-19 Monday night, according to the county health department's Facebook page.
County health officials are conducting contract tracing and an exposure investigation, and the individual has been isolated.
No other details have been released.
The Northern Cheyenne tribe announced that the case did not occur on its reservation, which partially extends into Rosebud County.
This new cases pushes the total cases in Montana to 521, with 43 active cases. This case, along with one in Big Horn County, have not been entered into the state's database.
MTN is reporting one fewer total case and one fewer recovered case due to a case in Jefferson County that local health authorities report involved a patient who was not in the state.