The name Oxalis oregana Nutt. is universally accepted for Redwood Sorrel despite a long history of attempted revisions. One of its many current synonyms, O. smalliana R.Knuth, was proposed over a century ago to describe a primarily Californian wood sorrel species distinguishable from the O. oregana type by flower size and color, but recognition of the taxon faded over time. The present study, however, provides substantial genetic, morphological, and phenological support for the reinstatement of O. smalliana as a separate species. A range-wide genetic study of 16 Redwood Sorrel populations showed divergence of two overlapping lineages at both nuclear (internal transcribed spacer) and chloroplast (psbJ-petA) loci. The southern lineage (ranging from Monterey County, California to Curry County, Oregon) corresponds in distribution and floral characteristics to previous descriptions of O. smalliana. The northern lineage, corresponding to O. oregana, ranges from southern Humboldt County, California to southwestern British Columbia. All O. smalliana specimens were characterized by apparent chloroplast heteroplasmy as evidenced by mixed sequence haplotypes, whereas all O. oregana specimens showed typical single sequence haplotypes. Flower size is generally larger in O. smalliana, and its flowers range in color from white to pink, purple, or blue, whereas O. oregana flowers are only white. Analysis of flower color distribution using more than 1000 images identified as O. oregana in the iNaturalist database showed a transition from southern pigmented flowers to northern white flowers in the Klamath Range of northwestern California and southwestern Oregon that correlated with the transition of the O. smalliana to O. oregana genetic lineages. In two sympatric populations, the white-flowered O. oregana was shown to attain peak flowering a full month later than the purple-flowered morph of O. smalliana. Phylogenetic analysis of internal transcribed spacer sequences showed species-level divergence of O. smalliana relative to O. oregana and other members of Oxalis subsect. Oxalis. Median-joining network analysis of haplotype sequences from all taxa in subsection Oxalis was confounded by probable hybridization and introgression, but provided some evolutionary insights. Divergence of the two species is discussed relative to the Pacific Northwest post-glacial north–south recolonization hypothesis.