The aim of this systematic review and meta-analysis was to assess (1) the effect of multiple-micronutrient (MMN) supplementation (MMS) vs iron and folic acid (IFA) supplementation among adolescents on maternal morbidity, birth outcomes, and mortality outcomes, (2) the effects of MMN supplementation in adolescents compared with the effects in adult women, and (3) the effect modification, if any, of MMN supplementation by baseline and geographic characteristics of adolescents. Thirteen randomized controlled trials conducted in Africa and Asia were identified from 1792 reviews and 1578 original trials. Results showed that in adolescents, MMS reduced low birth weight, preterm birth, and small-for-gestational-age births when compared with IFA supplementation. The effects of MMN supplementation did not differ between adolescents and older women, although a potentially greater reduction in small-for-gestational-age births was observed among adolescents. Effect modification by baseline characteristics and geographic region was inconclusive. Thus, MMS can improve birth outcomes among pregnant adolescents in low- and middle-income countries.