BACKGROUND: Children born to adolescent mothers are more vulnerable to infant mortality and morbidity than those born to adult mothers. HIV-exposed children have lower antibody protection against vaccine-preventable diseases at birth compared to unexposed children. In South Africa, 17 % of adolescent girls aged 15-19 years are mothers, yet vaccination coverage and timeliness among their children is underreported. METHODS: This study estimated age-appropriate vaccination coverage and timeliness among children (n = 1080) of adolescent mothers (n = 1015) in the Eastern Cape, South Africa. Mother-child dyads were recruited through healthcare and community-based sampling strategies. Vaccination data were abstracted from 1013 home-based child health records (2017-2019). Coverage is reported for Diphtheria-Tetanus-Pertussis 3rd dose (DTP3), under-1 vaccination among children over 12 months (n = 613) and measles 2nd dose (MCV2) among children over 24 months (n = 382) using proportions with 95 % confidence intervals (95 %CI). Timeliness is defined as receiving each vaccination within 4 weeks of recommended age. Findings are disaggregated by maternal HIV-status. RESULTS: Overall, 27.3 % of adolescent mothers were living with HIV. Coverage of DTP3 was 85.6 % (95 %CI: 82.6-88.3 %), under-1 coverage was 53.2 % (95 %CI: 49.1-57.2 %), and MCV2 coverage was 62.3 % (95 %CI: 57.2-67.2 %). Vaccination coverage was lower among children of adolescent mothers living with HIV (AMLHIV) than unexposed children (DTP3 80.3 % vs 88.2 % p-value: 0.01; under-1 46.5 % vs 56.4 % p-value: 0.02; MCV2 55.4 % vs 67.1 % p-value: 0.02). Timeliness of vaccinations declined over time from 98.0 % at birth, 70.7 % at 14 weeks, 71.9 % at 9 months and 37.3 % at 18 months. CONCLUSION: Vaccination coverage among children of adolescent mothers in the Eastern Cape are below national targets. Children of AMLHIV had lower coverage than HIV-unexposed children. Further research is needed to identify risk factors associated with incomplete and delayed vaccinations among this group, particularly among HIV-exposed children. Enhanced vaccination campaigns may be required for children of adolescent mothers.