The adverse consequences of untreated perinatal depression highlight the need to identify populations to target in order to increase treatment rates. The authors sought to evaluate treatment initiation for a new diagnosis of depression during pregnancy or postpartum and to describe racial-ethnic differences in initiation and type (psychotherapy, antidepressants) of treatment in a large health care system with universal perinatal depression screening. This retrospective cohort study included women who delivered a live birth in the Kaiser Permanente Northern California system between October 2012 and May 2017. Black, Latina, Asian, and White women ages ≥15 years were eligible. New depression diagnoses were defined by using ICD-9 and ICD-10 codes from electronic health records. Treatment initiation was defined as receiving at least one antidepressant medication dispensation or psychotherapy visit up to 90 days after the diagnosis. Modified Poisson regression was used to estimate the risk for initiating treatment and the type of treatment initiated. In total, 13,637 women with a new depression diagnosis (prenatal: N=7,041, 51.6%; postpartum: N=6,596, 48.4%) were identified. Of the pregnant women, 31.4% initiated treatment, and of the postpartum women, 73.1% initiated treatment. Latina and Asian women were less likely than White women to initiate treatment postpartum. During pregnancy and postpartum, non-White women were more likely to initiate psychotherapy. White women were more likely to initiate antidepressant medication during pregnancy and postpartum or a combination of antidepressant medication and psychotherapy during the postpartum period. Research is warranted to identify patient-, provider-, and system-level barriers that contribute to racial-ethnic disparities in perinatal mental health care.