It has been shown that racial/ethnic disparities exist with regard to initiation of and adherence to adjuvant endocrine therapy (AET). However, the relationship among American Indian/Alaska Native (AIAN) individuals is poorly understood, particularly among those who reside in urban areas. We evaluated whether AET initiation and adherence were lower among AIAN individuals than those of other races/ethnicities who were enrolled in the Kaiser Permanente of Northern California (KPNC) health system. We identified 23,680 patients from the period 1997 to 2014 who were eligible for AET (first primary, stage I-III, hormone receptor-positive breast cancer) and used KPNC pharmacy records to identify AET prescriptions and refill dates. We assessed AET initiation (≥1 filled prescription within 1 year of diagnosis) and AET adherence (proportion of days covered ≥80%) every year up to 5 years after AET initiation. At the end of the 5-year follow-up period, 83% of patients were AET initiators, and 58% were AET adherent. Compared with other races/ethnicities, AIAN women had the second-lowest rate of AET initiation (non-Hispanic Black [NHB], 78.0%; AIAN, 78.6%; Hispanic, 83.0%; non-Hispanic White [NHW], 82.5%; Asian/Pacific Islander [API], 84.7%), the lowest rate of AET adherence after 1 year and 5 years of follow-up (70.3% and 50.8%, respectively), and the greatest annual decline in AET adherence during the 4- to 5-year period of follow-up (a 13.8% decrease in AET adherence [from 64.6% to 50.8%]) after initiation of AET. In adjusted multivariable models, AIAN, Hispanic, and NHB women were less likely than NHW women to be AET adherent. At the end of the 5-year period, total underutilization (combining initiation and adherence) in AET-eligible patients was greatest among AIAN (70.6%) patients, followed by NHB (69.6%), Hispanic (63.2%), NHW (58.7%), and API (52.3%) patients, underscoring the AET treatment gap. Our results suggest that AET initiation and adherence are particularly low for insured AIAN women.