St. Marys is a city in Elk County, Pennsylvania, United States. The population was 12,738 at the 2020 census. It was founded December 8, 1842, and was originally inhabited mostly by Bavarian Roman Catholics. It is home to Straub Brewery and the first Benedictine convent in the United States. In 1992, the borough of St. Marys absorbed the surrounding township of Benzinger and incorporated as a city. Despite being Pennsylvania's second-largest city by area after Philadelphia, it is one of the state's least densely populated cities.