Introduction: We developed an 18‐month Happy Teen 2 (HT2) programme comprised of a one‐day workshop, two half‐day sessions, and three individual sessions to prepare HIV‐infected youth for the transition from paediatric to adult HIV care services. We describe the programme and evaluate the change in youth's knowledge scores.Methods: We implemented the HT2 programme among HIV‐infected Thai youth aged 14–22 years who were aware of their HIV status and receiving care at two hospitals in Bangkok (Siriraj Hospital, Queen Sirikit National Institute of Child Health [QSNICH]). Staff interviewed youth using a standardized questionnaire to assess HIV and health‐related knowledge at baseline and at 12 and 18 months while they participated in the programme. We examined factors associated with a composite knowledge score ≥95% at month 18 using logistic regression.Results: During March 2014–July 2016, 192 of 245 (78%) eligible youth were interviewed at baseline. Of these, 161 (84%) returned for interviews at 12 and 18 months. Among the 161 youth, the median age was 17 years, 74 (46%) were female, and 99% were receiving antiretroviral treatment. The median composite score was 45% at baseline and increased to 82% at 12 months and 95% at 18 months (P < 0.001). The range of median knowledge scores for antiretroviral management, HIV monitoring, HIV services, and family planning significantly increased from baseline (range 0–75%) to (range 67–100%) at 12 months and to 100% at 18 months (P < 0.001). Almost all youth were able to describe education and career goals at 12 and 18 months compared to 75% at baseline. In multivariable analysis, a composite knowledge score at 18 months >95% was associated with education level >high school (aOR: 2.15, 95%CI, 1.03–4.48) and receipt care at QSNICH (aOR: 2.43, 95%CI, 1.18–4.98). Youth whose mother and father had died were less likely to have score ≥95% (aOR: 0.22, 95%CI, 0.07–0.67) than those with living parents.Conclusions: Knowledge useful for a successful transition from paediatric to adult HIV care increased among youth participating in the HT2 programme. Youth follow‐up will continue to assess the impact of improved knowledge on outcomes following the transition to adult care services.