The Childhood Autism Spectrum Test (CAST; Scott, Baron‐Cohen, Bolton & Brayne, 2002) has proved to be a good test for screening autism spectrum disorders (ASD) and social communication problems. This study provides evidence on its psychometric properties, describe its validity for screening and provides data on its discriminative capabilities in a Spanish sample of 4–12 year‐old children from community and clinical settings (N = 1460 and 36, respectively). Factorial and convergent validity is also assessed. The full Spanish version with a cut‐off score of 15 presented a high sensitivity (83.9%) and specificity (92.5%), a positive predictive value (PPV) of .63 and an internal consistency (α) of .826. Some items showed low discriminating power and these results led us to propose a reduced version with 28 items and a cut‐off score of 13, which presented a high sensitivity (85.7%) and specificity (91.16%), a PPV of .61 and an α of .839. Correlations were high between the Spanish full and reduced versions of the CAST, and ADI‐R (Autism Diagnostic Interview Revised) and ADOS‐2 (Autism Diagnostic Observation Schedule, Second Edition). Like previous adaptation studies, a two‐factor structure was found. The data show that the CAST can be a valid and reliable questionnaire for ASD screening in Spanish clinical and community populations. Autism Res 2017, 10: 1491–1498. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.