ImportanceAdolescent suicide in the US is a major public health problem, yet temporal trends in suicide methods by demographics are understudied.ObjectiveTo examine national trends in suicide mortality by method (firearm, poisoning, hanging and asphyxiation, and all other means) from 1999 to 2020 by demographic characteristics.Design, Setting, and ParticipantsThis serial cross-sectional study used national death certificate data of adolescent (aged 10-19 years) suicide decedents compiled by the National Center for Health Statistics from January 1, 1999, to December 31, 2020. Data analysis was performed from April 1, 2023, to July 9, 2023.ExposuresAge, sex, and race and ethnicity.Main Outcomes and MeasuresTrends in age-standardized mortality rates and average annual percent change (AAPC) in rates were estimated by age, sex, and race and ethnicity for each suicide method.ResultsThis study assessed data from 47 217 adolescent suicide decedents. From 1999 to 2020, suicide by firearm (AAPC, 1.0; 95% CI, 0.1-1.9), poisoning (AAPC, 2.7; 95% CI, 1.0-4.4), hanging and asphyxiation (AAPC, 2.4; 95% CI, 0.2-4.6), and other means (AAPC, 2.9; 95% CI, 1.2-4.6) increased. Rapidly increasing rates were observed among female adolescents for poisoning (AAPC, 4.5; 95% CI, 2.3-6.7) and hanging and asphyxiation (AAPC, 5.9; 95% CI, 5.0-6.8) suicides. From 2007 to 2020, firearm suicides sharply increased among female (annual percent change [APC], 7.8; 95% CI, 6.0-9.5) and male (APC, 5.3; 95% CI, 4.3-6.3) adolescents. Firearm suicide rates increased among Black adolescents from 2012 to 2020 (APC, 14.5; 95% CI, 9.7-19.5), Asian and Pacific Islander adolescents from 2008 to 2020 (APC, 12.0; 95% CI, 9.7-14.5), American Indian and Alaska Native adolescents from 2014 to 2020 (APC, 10.6; 95% CI, 2.6-19.3), and Hispanic or Latino adolescents from 2011 to 2020 (APC, 10.2; 95% CI, 6.3-13.8). During the study period, Black adolescents had the highest average increase in hanging and asphyxiation suicides (AAPC, 4.2; 95% CI, 3.2-5.2). From 2011 to 2020, poisoning suicide deaths increased (APC, 12.6; 95% CI, 8.5-16.7) among female adolescents.Conclusions and RelevanceSuicide rates increased across all methods from 1999 to 2020. Differences were noted by sex, age, and race and ethnicity. Increasing suicide rates among racial and ethnic minoritized youth are especially concerning, and effective prevention strategies are urgently needed.