A new Afrotropical species of Polynema Haliday, 1833 (Hymenoptera: Mymaridae), Polynema (Polynema) sagittaria van Noort & Triapitsyn, sp. n., is described and illustrated based on specimens collected in the Cape Floral region in south-western South Africa. This species is morphologically closely related to the recently described Polynema (Polynema) dikobraz Triapitsyn, 2017 from Madagascar, both species possessing enlarged spine-like microtrichia on the fore wing disc that are unique among all the known world fairyflies. This new species belongs to the informal dikobraz species-group of the nominate subgenus of Polynema, which previously was only known from Madagascar. In addition, P. sagittaria has the ovipositor extending ventrally under the mesosoma to well in front of the head, in a bow-like curve, and housed in a narrow, anterior elongation of the metasoma, the basal sac. Occurrence and possible significance of such a bizarre ovipositor in other Mymaridae is discussed. All images and online keys are available on www.waspweb.org