Objectives: There is currently no booster diphtheria or tetanus vaccine for Lao children before adolescence, despite international recommendations. We investigated seroprotection against diphtheria and tetanus among Lao adolescents. Methods: Seven hundred seventy-nine serum samples were tested for anti-diphtheria and anti-tetanus antibodies. Results: Overall, 25.8% of the adolescents had antibody titers corresponding to protection against diphtheria and 30.9% to sufficient immunity against tetanus. Female participants >16 years were more likely to be protected against diphtheria (p < 0.001) and tetanus (p < 0.029). Conclusion: Low protection against diphtheria and tetanus, possibly due to low vaccination coverage or antibody waning, suggests booster doses are warranted before adolescence.