Sedenia polydesma is a moth in the Crambidae family. It is found in Australia, where it has been recorded from New South Wales. The wingspan is about 16 mm. The forewings are grey-whitish, sprinkled with ochreous-fuscous scales, which tend to form three moderate transverse fasciae. The hindwings are light fuscous, with two darker fuscous transverse fasciae and a fine line of fuscous scales along the hindmargin.