We longitudinally explored frequency of disordered eating among transgender and nonbinary (TGNB) adolescents and explored trends by gender identity and gender-affirming care. Participants completed an abbreviated version of the Eating Disorder Examination Questionnaire (EDE-Q) at baseline, 3, 6, and 12 months after establishing care in a gender clinic. We analyzed descriptive statistics and multivariate linear regression analyses. Of the 91 TGNB adolescent participants, 61% were transmasculine, 30% transfeminine, and 7% nonbinary/gender-fluid. Among TGNB adolescents, disordered eating thoughts/behaviors were frequently endorsed with 26% of participants engaging in any occurrence of binge eating, 27% limiting the amount of food they ate, and 30% excluding foods from their diet. Forty percent of participants reported any occurrence of at least 1 disordered eating behavior and 17% at least 3 behaviors. Abbreviated EDE-Q responses did not differ significantly by sex assigned at birth, gender identity, gender-affirming medications, or time spent receiving gender-affirming care. There was a significant effect of age (P value=.003) on abbreviated EDE-Q scores. There were no significant changes in disordered eating after initiating gender-affirming medical care, possibly due to the limited study time frame of 12 months. Given the high prevalence of disordered eating behaviors, clinicians should consider screening all TGNB adolescents for disordered eating thoughts/behaviors throughout gender-affirming care. Future longitudinal research should recruit larger samples with a diverse range of gender identities and survey disordered eating thoughts/behaviors at least one year after starting gender-affirming medications.