Importance: Receipt of hepatitis B virus vaccine is important to prevent infection. However, adherence to the hepatitis B vaccine series among adults at risk of infection has been low. Objective: To assess whether recipients of a 2-dose hepatitis B vaccine with cytosine phosphoguanine adjuvant (HepB-CpG vaccine; Heplisav-B) are more likely to complete their series compared with recipients of a 3-dose vaccine with alum adjuvant (comparator vaccine; Engerix-B [HepB-alum]). Design, Setting, and Participants: This nested cohort study was conducted from August 7 to December 31, 2018, at Kaiser Permanente Southern California, an integrated health care system with a diverse population of approximately 4.6 million members. Adults not receiving dialysis who received a first dose of a hepatitis B vaccine series in family practice or internal medicine departments of 15 Kaiser Permanente Southern California medical centers were followed up through electronic health records for up to 1 year after receipt of the first dose. Data were analyzed from March 16 to September 23, 2020. Exposures: Receipt of a first dose of the HepB-CpG vaccine (2-dose vaccine) vs receipt of a first dose of the HepB-alum vaccine (3-dose vaccine). Main Outcomes and Measures: Series completion within the recommended vaccine schedule plus 3 months (primary outcome) and series completion within 1 year after receipt of the first dose (secondary outcome). Results: Of 4727 individuals who initiated the HepB-CpG vaccine series and 6161 individuals who initiated the HepB-alum vaccine series included in the study, 2876 (60.8%) and 3789 (61.5%), respectively, were ages 40 to 59 years, 2415 (51.1%) and 3113 (50.5%) were male, and 2364 (50.0%) and 2881 (46.8%) were Hispanic. The vaccine series was completed within the recommended schedule plus 3 months for 2111 (44.7%) individuals who initiated the HepB-CpG vaccine series and 1607 (26.1%) individuals who initiated the HepB-alum vaccine series, and within 1 year for 2858 (60.5%) and 1989 (32.3%) individuals, respectively. The individuals who initiated the HepB-CpG vaccine series were significantly more likely to complete the series (adjusted relative risk, 1.77; 95% CI, 1.68-1.87). Results were consistent across clinical and demographic strata. Conclusions and Relevance: In this study, use of the HepB-CpG vaccine was associated with hepatitis B vaccine series completion, but tailored strategies to increase completion of hepatitis B vaccine series are warranted.