BackgroundUnintended pregnancy at a young age can lead to poor reproductive health, social and economic outcomes. The high rate of unintended teenage pregnancies in Tanzania is indicative of inadequate availability and uptake of modern contraception. Determining trends and determinants of unmet need for modern contraception among adolescent girls and young women (AGYW) in Tanzania will help address the burden of unintended pregnancies.MethodologyAn analytical cross-sectional study design was conducted using secondary data from three consecutive Tanzania Demographic and Health Surveys (TDHS) 2004/05, 2010 and 2015/6. Data analysis was performed using Stata version 15.0. Data analysis considered the complex survey design. Categorical and continuous variables were summarized using descriptive statistics. Poisson regression model was used to determine factors associated with unmet need for modern contraception.ResultsA steady decline in unmet need for modern contraception was observed from 31.8% in 2004/05 to 27.5% in 2015/16 survey. In the multivariable analysis, higher prevalence of unmet need for modern contraception was observed among adolescents, participants with at least one live birth, from poor wealth tertile, and those sexually active during the past four weeks.ConclusionDespite declining levels, the unmet need for modern contraception among young women in Tanzania remains high. AGYW under 19 years, those from poor households, and those who are postpartum are most at risk. Greater efforts are required to meet the reproductive health needs and rights of these sub-groups of AGYW in order to facilitate uptake of modern contraceptives and therefore reducing the risk of unintended pregnancies and unmet need for modern contraception.