Preventing unwanted pregnancy through adequate use of contraceptives among adolescent girls is essential for improving maternal and child health and social well-being. To estimate the levels and trends of contraceptive use in adolescent girls (age 15-19 years) compared with adult women (age 20-34 years). This survey study used data from 261 Demographic and Health Surveys or Multiple Cluster Indicator Surveys conducted in 103 low- and middle-income countries between 2000 and 2017. Modern contraceptive use in 90 countries and unmet need for family planning in 73 countries for adolescent girls and adult women were assessed at country and aggregate levels. Data analyses for this study were conducted between March 2019 and December 2019. Modern contraceptive use and unmet need for family planning, estimated between adolescent girls and adult women for absolute inequality and relative inequality (defined as the ratio in service use between adult women and adolescent girls). Inequalities by wealth and place of residence in each age group were also assessed. A total of 832 673 adolescent girls and 2 156 268 adult women were included in the analysis. Adolescent girls were significantly less likely to use modern contraceptives (31.6% [95% CI, 30.3% to 32.8%] vs 43.5% [95% CI, 42.4% to 44.7%]; absolute inequality, 11.9 percentage points [PPs] [95% CI, 11.7 to 12.1 PPs]; relative inequality, 1.38 PPs [95% CI, 1.36 to 1.40 PPs]) and more likely to have an unmet need for family planning (50.8% [95% CI, 49.0% to 52.5%] vs 36.4% [95% CI, 34.9% to 35.8%]; absolute inequality, -14.4 PPs [95% CI, -15.8 to -15.0 PPs]; relative inequality, 0.72 PPs [95% CI, 0.69 to 0.75 PPs]). Contraceptive use generally improved over time (aggregate-level use of contraceptives increased from 17.8% [95% CI, 16.6% to 19.0%] in 2000-2006 to 27.2% [95% CI, 26.6% to 27.8%] in 2013-2017 for adolescents and from 30.9% [95% CI, 29.8% to 32.0%] in 2000-2006 to 40.3% [95% CI, 39.8% to 40.8%] in 2013-2017 for adult women), but inequalities between adolescents and adult women persisted. Aggregate-level prevalence of unmet need for family planning decreased only among adult women (from 45.8% [95% CI, 44.9% to 46.7%] in 2000-2006 to 38.0% [95% CI, 37.3% to 38.7%] in 2013-2017). For adolescents, the prevalence remained at approximately 52% over time. The inequality between the 2 age groups therefore significantly increased from 7.5 PPs (95% CI, 6.5 to 8.4 PPs) in 2000 to 2006 to 14.0 PPs (95% CI, 13.4 to 14.6 PPs) in 2013 to 2017. An increasing gap between adolescent girls and adult women was found in 18 countries for modern contraceptive use and in 20 countries for unmet need for family planning. In India, for example, both age groups had increased percentage of unmet need for family planning over time, from 16.2% (95% CI, 15.9% to 16.6%) in 2006 to 29.8% (95% CI, 29.6% to 30.1%) in 2015 for adult women and from 23.9% (95% CI, 23.0% to 24.9%) to 64.5% (95% CI, 63.3% to 65.7%) among adolescents. Thus, inequality increased from 7.7 PPs (95% CI, 7.2 to 8.2 PPs) in 2006 to 34.7 PPs (95% CI, 34.2 to 35.1 PPs) in 2015. Despite a growing focus on contraception in the past decade with initiatives such as Family Planning 2020, adolescent girls continue to lag behind adult women in contraceptive use. There is a pressing need to develop age-appropriate strategies for increasing access and maintaining contraceptive use in sexually active adolescent girls.