We assessed factors associated with clinical, social, and behavioral outcomes of adolescents and young adults with HIV (AYHIV) in Southeast Asia after transition from pediatric to adult HIV care. AYHIV in Malaysia, Thailand, and Vietnam were prospectively followed through annual clinical assessments and laboratory testing. Data were described descriptively and a generalized estimating equation was used to calculate independent predictors for HIV viremia (>40 copies/mL). A total of 93 AYHIV were followed until February 2019: 60% female, 94% acquired HIV perinatally, 81% Thai, median age 20 (interquartile range, 18-21) years. The median follow-up time was 94 (91-100) weeks; 88% completed the study. At week 96, median CD4 was 557 cells/mm3 (interquartile range, 337-786), 77% had suppressed HIV viral load, 39% reported recent alcohol use, 49% had been sexually active, 53% of females and 36% of males intended to have children, and 23% screened positive for moderate depression (Patient Health Questionnaire-9 score ≥9) or reported suicidal ideation. HIV viremia was associated with <90% adherence to HIV treatment (adjusted incidence rate ratio [aIRR] 2.2 [1.28-3.78]), CD4 count ≤500 cells/mm3 (aIRR 4.75 [2.11-10.69]), and being on a nonnucleoside reverse transcriptase inhibitor regimen (vs. protease inhibitor aIRR 2.71 [1.13-6.49]). Having a trusted person to talk with about their feelings was protective (vs. never; usually or always, aIRR 0.41 [0.18-0.92]). After transition to adult HIV care, there were indications of social isolation and mental health problems that could prevent these AYHIV from maintaining control over their HIV infection and hinder progress toward social independence.