•Initiation rate rose after implementation of 1st-dose reminder at intervention site.•Completion rate rose after introduction of all-dose alert at intervention site.•During the same period, the corresponding rates at control site remained stable. The Advisory Committee on Immunization Practices recommends Hepatitis B (HepB) vaccine for previously unvaccinated adults <60 years with diabetes mellitus. This observational retrospective cohort study assessed the impact of implementing electronic provider reminders on HepB vaccine initiation and 3-dose series completion rates among insured adults with diabetes aged 19–59 years old. Difference-in-difference (DID) analyses compared changes in vaccine initiation and completion rates (ratio of the rate ratio RRR and 95% confidence interval CI) during 12 months pre- and post-implementation between intervention and control sites. We examined trends in vaccine initiation and completion rates by plotting monthly rates during the study period. We also calculated the overall HepB vaccine coverage rates with 95% CI among all adults with diabetes aged 19–59 years old at the start and end date of the study period. Baseline HepB vaccine initiation and completion rates were similar at both the intervention and control sites. Gender, age, and race/ethnicity distributions within both sites were similar during the 12 months pre- and post-implementation. DID analyses demonstrated statistically significant differences in the changes of the annual vaccine initiation rates (RRR: 70.7, 95% CI: 62.8–79.6) and the third dose completion rates (RRR = 18.7, 95% CI: 14.2–24.8) between the two sites. The coverage increased significantly at the intervention site while it remained low at the control site. Use of provider reminders is highly effective in increasing both HepB vaccine initiation and series completion rates among adults with diabetes.