Saltfleetby-Theddlethorpe Dunes is a national nature reserve on the coast of Lincolnshire, England, in the parishes of Saltfleetby and Theddlethorpe. It is managed in part by Natural England; in part by the Lincolnshire Wildlife Trust, and consists of 1,377 acres (557 ha) of sand dunes, salt marsh, sand and mudflats. It is on Rimac Road and sometimes referred to as Rimac.