Xiombarg is a monotypic genus of spiders in the family Oonopidae containing the single species, Xiombarg plaumanni, occurring in southeastern Brazil and adjacent Argentina.
Italian arachnologist Paolo Marcello Brignoli described X. plaumanni in 1979, naming the genus after Xiombarg, a character in Michael Moorcock novels. The holotype was collected by F. Plaumann.
Xiombarg plaumanni has been recorded from Brazil and Argentina.