This study estimated the prevalence of autism spectrum disorder in a 5-year inpatient cohort of children and adolescents diagnosed with first-episode psychosis in two Hong Kong teaching hospitals, and explored the clinical characteristics predicting first-episode psychosis-autism spectrum disorder comorbidity. The autism spectrum disorder diagnoses of 103 patients with first-episode psychosis were confirmed by Autism Diagnostic Interview-Revised. Based on the (1) "Diagnostic and Statistical Manual of Mental Disorders" (5th ed.) diagnostic checklist; and (2) Autism Diagnostic Interview-Revised, three major observations were reported: (1) the prevalence of autism spectrum disorder among children and adolescents with first-episode psychosis was 28.2% (95% confidence interval = [19.7%, 37.9%]); (2) after administering the Autism Diagnostic Interview-Revised, an additional 31.0% of first-episode psychosis patients met the clinical diagnostic cut-off for autism spectrum disorder; and (3) multivariate binary logistic regression analysis showed that first-episode psychosis patients with a comorbid autism spectrum disorder diagnosis were more likely to be a male than a female (adjusted odds ratio = 3.57, 95% confidence interval = (1.29, 9.86), p = 0.014). Adding to the current literature, this study examined first-episode psychosis-autism spectrum disorder comorbidity with a large Chinese, inpatient sample with first-episode psychosis. Consistent with studies conducted overseas, we show that autism spectrum disorder is a common comorbidity among Chinese first-episode psychosis patients. A comprehensive autism spectrum disorder assessment that incorporates both observation-based and parent-report diagnostic tools is warranted for children and adolescents, especially boys, with psychosis.