BACKGROUND: Differences in social behaviours are common in young people with neurodevelopmental conditions (NDCs). Recent research challenges the long-standing hypothesis that difficulties in social cognition explain social behaviour differences. AIMS: We examined how difficulties regulating one's behaviour, emotions and thoughts to adapt to environmental demands (i.e. dysregulation), alongside social cognition, explain social behaviours across neurodiverse young people. METHOD: We analysed cross-sectional behavioural and cognitive data of 646 6- to 18-year-old typically developing young people and those with NDCs from the Province of Ontario Neurodevelopmental Network. Social behaviours and dysregulation were measured by the caregiver-reported Adaptive Behavior Assessment System Social domain and Child Behavior Checklist Dysregulation Profile, respectively. Social cognition was assessed by the Neuropsychological Assessment Affect-Recognition and Theory-of-Mind, Reading the Mind in the Eyes Test, and Sandbox continuous false-belief task scores. We split the sample into training ( RESULTS: Two social cognition components adequately explained social behaviours (13.88%). Lower dysregulation further explained better social behaviours ( CONCLUSIONS: Self-regulation, beyond social cognition, substantially explains social behaviours across neurodiverse young people.