The Swamp Antechinus is endemic to Australia. The subspecies A. m. maritimus (Finlayson 1958) is found as fragmented populations in near-coastal areas of south-eastern South Australia and Victoria west of and including Sunday Island and Wilsons Promontory. Isolated populations in southern Gippsland Plain have been most affected by habitat modification and/or alienation. In South Australia, recorded since 1970 from coastal lakes and swamps south of Robe. In Victoria, recorded since 1970 from three main areas, the south-western Wannon region, the Otways, and the southern Gippsland Plain (Menkhorst 1995; Bachmann and van Weenen 2001).

A. m. minimus (Geoffroy Saint-Hilaire 1803) is present on Tasmania and the Bass Strait Islands (Wilson and Bachmann 2008).