231st

231st may refer to:


 * 231st Battalion (Seaforth Highlanders of Canada), CEF, a unit in the Canadian Expeditionary Force during the First World War
 * 231st Combat Communications Squadron, a tenant unit of the 113th Wing based at Andrews Air Force Base, Maryland, USA
 * 231st Infantry Brigade, the Allied codename for the centre invasion beach during the World War II Allied invasion of Normandy, 6 June 1944