Artioposthia diemenensis, also known as Van Diemen's flatworm, is a species of land planarian in the tribe Caenoplanini. It has been found in southern Australia, including Tasmania.
Artioposthia diemenensis is generally flat and broad, sloping inwards at its ends. When crawling, it can reach up to 70 mm in length and 6 mm in width. The backside is a sepia brown color with some darker stripes running down; the entire backside is covered in whitish speckles. The underside is a whitish color.