The Modified Checklist for Autism in Toddlers-revised/follow-up (M-CHAT-R/F) was developed to reduce the number of cases requiring telephone verification. The aim of this study was to validate a Spanish version of the M-CHAT-R/F in the Spanish public health system. The M-CHAT-R/F was translated, culturally adapted, and then administered to 6625 children. Of the 39 positive screening cases, 15 children were diagnosed with autism spectrum disorder (ASD) and 24 with non-ASD disorders or delays. The sensitivity was 0.79 and specificity of 0.99. Positive and negative predictive values were 0.39 and 0.99, respectively. These results are similar to the English equivalent, though observed prevalence was lower. This study supports Spanish National Health System policy makers to consider a universal ASD screening program.