HIV infections among adolescents and young adults continue to grow and clinical guidelines recommend the immediate start of life-saving antiretroviral therapy (ART). Unfortunately, suboptimal medication adherence among youth is common and can lead to poorer health outcomes as well as onward transmission of HIV to sexual partners. Clinical tools to assess treatment readiness are needed and can assist with adherence intervention strategies for youth. An assessment tool that we previously developed, the HIV Treatment Readiness Measure (HTRM), was administered to 595 HIV-positive youth ages 13–24 recruited from adolescent medicine clinics in the United States. Participants were followed for a minimum of 6 months and had to have at least one viral load test completed to be included in this analysis. The HTRM demonstrated high internal consistency (Chronbach’s alpha = 0.86). For participants currently on ART at study entry, higher overall treatment readiness scores predicted future viral suppression (OR 1.52). Individual scores on three of the measure’s factors (Psychosocial Issues, Connection with Care, and HIV Medication Beliefs) were also significant predictors of viral suppression. For those participants not on ART at study entry, the HIV Medication Beliefs factor significantly predicted who would eventually start ART (OR 2.26) but overall treatment readiness scores did not predict viral suppression in that group.