Altmar is a hamlet in Oswego County, New York, United States. The population was 407 at the 2010 census.The former Village of Altmar is in northern part of the Town of Albion at State Route 13 and County Route 22. Altmar is the seat of the town government.As of 2016, the U.S. Census Bureau continues to list Altmar as a village. It will soon be redefined as a CDP.