St. Mary's is a city in Kusilvak Census Area, Alaska, United States. The adjacent village of Andreafsky joined with St. Mary's in 1980. At the 2010 census the population was 507, up from 500 in 2000. By 2018, the population was estimated to be 567. Within Saint Mary's there are two federally-recognized tribes the Algaaciq Native Village and the Yuupiit of Andreafsky.