The Old Montana Prison Museum is a historic site and museum in Deer Lodge, Montana. It features the former Montana State Prison, which is a very historic structure in the state.

Built in 1871, the Montana State Prison served as the Montana Territorial Prison from its completion and until 1979, when it was replaced due to its continuous degeneration with the current facility. It is also the very site where the 1959 nationally known riot took place where the Montana National Guard had to step in to restore order.

The Old Montana Prison Museum now serves as a museum owned by the city of Deer Lodge and operated by the Powell County Museum and Arts Foundation. It has been featured in several films and TV shows in the past.