This technical brief from MOMENTUM Country and Global Leadership explores the link between adolescent sexual and reproductive health (SRH) and perinatal mental health (PMH) in low- and middle-income countries, highlighting the particular vulnerabilities faced by adolescent girls. It also presents key findings from a landscape analysis and literature review, indicating that mental health conditions among adolescents increase the likelihood of unintended pregnancies, which in turn increase the risk of PMH conditions. The document also discusses the impact of PMH conditions on contraceptive use and future unintended pregnancies and highlights promising approaches to improve adolescent PMH and related SRH outcomes, emphasizing the need for further research in this area.