A new fern species, Polystichum oblanceolatum H. He & Li Bing Zhang (Dryopteridaceae), is described and illustrated from a karst cave in Guangxi, southern China. As a member of Polystichum Roth sect. Haplopolystichum Tagawa, P. oblanceolatum is morphologically most similar to P. dielsii Christ and P. liui Ching, but is distinguishable from the latter two by its often oblanceolate lamina with acute apex, its repand or broadly dentate pinnae with non-spinulose toothed distal margins, and the less developed, more or less truncate, but not sharply acute, basal acroscopic auricles of the pinnae. The new taxon appears to be endemic to a single karst cave in Tian'e County in northern Guangxi and is considered to be Critically Endangered (CR), based on IUCN Red List criteria. A key is provided to facilitate the identification of P. oblanceolatum and morphologically similar species.