Lepechinia rossii (Ross’s pitcher sage) is a highly aromatic shrub narrowly endemic to southern California. It occurs in chaparral communities at two locations in the Western Transverse Ranges of Los Angeles and Ventura counties. It is listed by the California Native Plant Society (CNPS) with a Rare Plant Rank of 1B.2, rare or endangered in California and elsewhere, and fairly endangered in California. It is considered “Critically Imperiled” both statewide (S1) and globally (G1; NatureServe 2020). It faces a range of threats from invasive species; changes in fire regimes; and human activities including off highway vehicle use, power line maintenance, and petroleum exploration. Conserving the handful of occurrences that are known to be extant is crucial to ensuring the future of this species. Conservation actions include resurveying known occurrences, searching for new occurrences and potential habitats, updating threat assessments, and studying the natural history of the species and genetic structure of known populations. Field work was conducted on in the Angeles National Forest (ANF) and Los Padres National Forest (LPNF) to sample leaf tissue from Lepechinia rossii populations across its range. Seventy-three samples were collected from the ANF and forty-five samples were collected from the LPNF. Many plants, especially in LPNF, had begun to lose their mature leaves due to drought conditions, so tissue for DNA extraction was taken from young leaves and protected buds. DNA has been extracted from 132 samples of L. rossii and outgroups including: L. hastata, L. calycina, L. fragrans, L. ganderi, and two samples of wild collected L. cardiophylla from the Santa Ana mountains. DNA was extracted using Qiagen DNeasy kits, and DNA concentrations were standardized to 20 ng/ul. A maximum likelihood phylogeny has been estimated with 1000 ultrafast bootstraps in IQtree on alignment of ddradseq 74,898 loci generated through an ipyrad pipeline. The preliminary phylogeny of sect. Calycinae of Lepechinia (Lamiaceae), with five sampled species from California (L. calycina, L. cardiophylla, L. fragrans, L. ganderi, and L. rossii) show a monophyletic group containing all sampled L. rossii as sister to a clade containing the remaining four California species. Furthermore, specimens from the two sampled localities (Ventura and Los Angeles counties) form reciprocally monophyletic groups. Population genomic analyses sampled the entire L. rossii range and nearby populations of L. fragrans and revealed highly differentiated populations that show little sign of admixture. Genetic structure of L. rossii populations in uncovered in this study will inform future conservation management and possible restoration efforts.