Nemesia diffusa is a perennial growing to 45 cm. Its opposite leaves are three to five-veined.
The flowers are white with dark purple line markings. The upper lip has four almost oblong lobes, their tips rounded; the lower lip is larger, broad with rounded tip and tapering to the base where it joins the corolla tube. This lower lip has a couple of bulging patches in its throat. Some flower forms with yellow raised patches in the throat also occur. There is a spur at the back of the corolla. The calyx is five-lobed. Flowering happens from spring to mid-autumn.
The distribution in the Western Cape ranges from Paarl to Worcester and the Caledon area, reaching the coast at Hermanus. This flower was found in Fernkloof during October. The plant grows on sandstone mountain slopes (Bond and Goldblatt, Bean and Johns, 2005; iSpot).