OBJECTIVES: To describe differences in modern contraceptive use among adolescent women aged 15-19 years according to their marital status and to determine factors associated with modern contraceptive use among sexually active women in this population. DESIGN: Cross-sectional analysis of Adolescent 360 evaluation baseline survey. SETTING: The 15 urban and semiurban wards of Ilemela district, Mwanza region, North-Western Tanzania. PARTICIPANTS: Adolescent women aged 15-19 years who were living in the study site from August 2017 to February 2018 and who provided informed consent. Women were classified as married if they had a husband or were living as married. Unmarried women were classified as sexually active if they reported having sexual intercourse in the last 12 months. OUTCOME MEASURE: Prevalence of modern contraceptive among adolescent women aged 15-19 years. RESULTS: Data were available for 3511 women aged 15-19 years, of which 201 (5.7%) were married and 744 (22.5%) were unmarried-sexually active. We found strong evidence of differences in use of modern contraceptive methods according to marital status of adolescent women. Determinants of modern contraception use among unmarried-sexually active women were increasing age, increasing level of education, being in education, hearing of modern contraception from interpersonal sources or in the media in the last 12 months, perceiving partner and/or friends support for contraceptive use, as well as higher knowledge and self efficacy for contraception. CONCLUSIONS: Sexual and reproductive health programmes aiming to increase uptake of modern contraceptives in this population of adolescent women should consider the importance of girl's education and social support for contraceptive use particularly among unmarried-sexually active women.