The purpose of this study was to examine the impact of a home-based diabetes prevention and management program on high-risk American Indian youth.Together on Diabetes (TOD) was designed via a participatory approach with 4 tribal communities in the southwestern United States. A multisite pre- and postevaluation design was used to evaluate the efficacy of the TOD intervention on improving youth's psychosocial, knowledge, behavioral, and physiological outcomes at 4 time points from baseline to 12 months postenrollment.A total of 256 youth and 225 support persons were enrolled in the TOD program. At 12 months postenrollment, improvements were observed in youth's quality of life (P < .001), depressive symptoms (P < .001), knowledge related to TOD content (P < .001), standardized body mass index scores (P = .004), and hypertension (P = .026). Improvements in mean A1C were observed among diabetic youth with baseline A1C >6.5% (P = .036).The TOD program was feasible, acceptable, and effective in lowering diabetes risk among reservation-based American Indian youth. It is the first efficacious youth-focused diabetes prevention and management program developed and implemented in partnership with tribal communities.