I report a new species of Nemophila growing sympatrically in some localities with N. menziesii Hook. & Arn. var. atomaria (Fisch. & Mey.) H. P. Chandler (sensu Constance 1941) in Northern California. The new species, N. hoplandensis, differs from N. menziesii var. atomaria in both floral and vegetative color. The corollas of N. menziesii var. atomaria range from deep blue to a bluish white. Nemophila hoplandensis has large and unusually bright white corollas and vegetative structures that are a brighter green than those of N. menziesii var. atomaria. I provide support from controlled crosses for treating these populations as a new species; despite sharing pollinators N. hoplandensis is reproductively isolated from N. menziesii var. atomaria via failure of hybrid seed production following artificial crosses. Molecular phylogenetic analyses also clearly differentiate N. hoplandensis within the genus. The newly described species, currently known only from Mendocino and Napa counties, has a range that is more restricted than and lies within the range of N. menziesii var. atomaria.