Lomandra marginata T.D.Macfarl. & Conran is described as a new species and illustrated by photographs and a distribution map. Recognised initially from vegetative plants during a biological survey at Lochada Station, it was realised subsequently that the species is widespread from Geraldton to Corrigin. However, it apparently flowers only occasionally under favourable conditions, so earlier collections were mostly vegetative. Lomandra marginata is placed in L. series Sparsiflorae on the basis of possessing a narrow floral bract that does not encircle the pedicel and a bracteole, when present, offset rather than opposite the bract. It has distinctive membranous leaf margins resembling those of the related genus Chamaexeros and the longest pedicels in the genus, mostly 10–16 mm long, on which are borne white flowers resembling those of L. effusa (Lindl.) Ewart. Although L. effusa is the most similar species in flower and inflorescence and is also a member of L. series Sparsiflorae, it differs vegetatively by having distinctly two-toothed leaf tips.