Background: Psychoeducation is an essential component of postdiagnostic care for people with ASD (autism spectrum disorder), but there is currently no evidence base for clinical practice. We designed, manualised and evaluated PEGASUS (psychoeducation group for autism spectrum understanding and support), a group psychoeducational programme aiming to enhance the self-awareness of young people with ASD by teaching them about their diagnosis. Methods: This single-blind RCT (randomised control trial) involved 48 young people (9-14 years) with high-functioning ASD. Half were randomly assigned to PEGASUS, administered in six weekly group sessions, with the others receiving no additional intervention. ASD-related self-awareness, the primary outcome, was evaluated using the bespoke Autism Knowledge Quiz (AKQ). Secondary outcome measures included the Rosenberg Self-Esteem Scale. All measures were collected during home visits and scored by researchers blind to group assignment. The trial is registered on ClinicalTrials (NCT01187940, http://www.clinicaltrials.gov) and was funded by the Baily Thomas Charitable Trust. Results: Bootstrap multiple regression showed ASD knowledge (ß = 0.29, p < 0.001, 95% CIs [0.13, 0.44]) and ASD self-awareness (ß = 0.42, p = 0.001, 95% CIs [0.17, 0.67]), measured by number of ASD-related personal strengths and difficulties listed by participants, increased for those who attended PEGASUS (n = 24) compared with controls (n = 24). There was no effect of PEGASUS on self-esteem by self-report (ß = 0.10, p = 0.404, 95% CIs [-0.14, 0.35]) or parent report (ß = 0.12, p = 0.324, 95% CIs [-0.12, 0.36]). Conclusions: After PEGASUS, participants had more general knowledge about ASD, and showed a greater awareness of their collection of unique strengths and difficulties associated with ASD. Psychoeducation did not lower self-esteem. This RCT provides initial evidence for PEGASUS's efficacy as a psychoeducation programme for people with ASD.