Silene nelsonii M.R. Mesler, M.S. Mayer, and S.K. Carothers (Caryophyllaceae), a species with large, day-blooming flowers from the Trinity River Basin of northwestern California, is described and illustrated. In the past, plants assignable to this new species have been referred to incorrectly as S. bolanderi A.Gray or S. hookeri Nutt. subsp. bolanderi (Gray) Abrams, names that rightfully apply to another member of the Silene hookeri complex. Here we argue that both S. nelsonii and S. bolanderi are worthy of recognition as separate species based on morphological distinction and a molecular phylogenetic analysis. Silene nelsonii differs from S. bolanderi by its more deeply lobed, white petals; lack of well-developed coronal petal appendages; densely ciliate petal bases; and shorter, sometimes branched sepal hairs. We present a key to the five members of the S. hookeri complex, all of which are in some degree rare in California and worthy of conservation concern.