Background <p>Almost one-tenth of all pregnancies and births worldwide are among adolescent women 15–19 years of age, and 90% of these births occur in developing countries. Adolescent mothers face risks related to the transition to adulthood and parenthood, physical changes, and potential health complications associated with early pregnancy and childbirth. These are associated with an increased risk for mental disorders. However, we have very limited information on the prevalence of and factors associated with depressive and generalized anxiety symptoms among pregnant adolescents from low-and-middle-income countries including Tanzania, a gap this study aimed to fill.</p> Methods <p>An analytical cross-sectional study was conducted among 529 adolescent pregnant women. A convenience sampling strategy was used to recruit participants from preselected antenatal care clinics (ANCs) from June 2023 to March 2024. The Edinburgh Postnatal Depression Scale (EPDS) was used to assess depressive symptoms and the Generalized Anxiety Disorder-7 (GAD-7) scale was used to screen for generalized anxiety symptoms. Trained research assistants interviewed participants and administered these questionnaires.</p> Results <p>The prevalence of depressive symptoms was 20.98%, and 22.5% of the adolescent women displayed symptoms of generalized anxiety disorder. Marital status, living with parents/caretakers, planned pregnancy, partner violence, and pregnancy related stigma were identified as factors associated with both depressive and generalized anxiety symptoms.</p> Conclusions <p>At least one of five adolescent women attending ANCs in Mwanza, Tanzania had symptoms of perinatal depression and/or generalized anxiety disorder. This supports the importance of integrating mental health services into existing antenatal and postnatal care services.</p>