We describe the results of a pilot randomized clinical trial of a mobile phone-based intervention, InTSHA: Interactive Transition Support for Adolescents with HIV, compared to standard care. Encrypted, closed group chats delivered via WhatsApp provided peer support and improved communication between adolescents with HIV, their caregivers, and healthcare providers. We randomized 80 South African adolescents ages 15 to 19 years with perinatally-acquired HIV to receive either the intervention (n=40) or standard of care (n=40). We measured acceptability (Acceptability of Intervention Measure [AIM]) and feasibility (Feasibility of Intervention Measure [FIM]) as primary outcomes. We evaluated impact on retention in care and viral suppression six months after randomization as secondary endpoints. We performed bivariable and multivariable analyses using logistic regression models to assess the effect of the InTSHA intervention compared to standard of care. Among the adolescents randomized to the InTSHA intervention, the median AIM was 4.1/5.0 (82%) and median FIM was 3.9/5.0 (78%). We found no difference in retention in care or in viral suppression comparing intervention and control groups. Among adolescents who attended three or more sessions, retention in care was 100% at 6 months. InTSHA is an acceptable and feasible mHealth intervention warranting further study in a larger population.