Objectives: One of Africa's most important challenges is to improve maternal and neonatal health. The identification of groups at highest risk for adverse pregnancy outcomes is important for developing and implementing targeted prevention programmes. This study assessed whether young adolescent girls constitute a group at increased risk for adverse birth outcomes among pregnant women in sub-Saharan Africa. Setting: Data were collected prospectively as part of a large randomised controlled clinical trial evaluating intermittent preventive treatment of malaria in pregnancy (NCT00811421—Clinical Trials.gov), conducted between September 2009 and December 2013 in Benin, Gabon, Mozambique and Tanzania. Participants: Of 4749 participants, pregnancy outcomes were collected for 4388 deliveries with 4183 live births including 83 multiple gestations. Of 4100 mothers with a singleton live birth delivery, 24% (975/4100) were adolescents (≤19 years of age) and 6% (248/4100) were aged ≤16 years. Primary and secondary outcome measures: Primary outcomes of this predefined analysis were preterm delivery and low birth weight. Results: The overall prevalence of low birthweight infants and preterm delivery was 10% (371/3851) and 4% (159/3862), respectively. Mothers aged ≤16 years showed higher risk for the delivery of a low birthweight infant (OR: 1.96; 95% CI 1.35 to 2.83). Similarly, preterm delivery was associated with young maternal age (≤16 years; OR: 2.62; 95% CI 1.59 to 4.30). In a subanalysis restricted to primiparous women: preterm delivery, OR 4.28; 95% CI 2.05 to 8.93; low birth weight, OR: 1.29; 95% CI 0.82 to 2.01. Conclusions: Young maternal age increases the risk for adverse pregnancy outcomes and it is a stronger predictor for low birth weight and preterm delivery than other established risk factors in sub-Saharan Africa. This finding highlights the need to improve adolescent reproductive health in sub-Saharan Africa.