The autism spectrum disorder prevalence data for southern Europe seem to be lower than international reports. The objective of the Neurodevelopmental Disorders Epidemiological Research Project was to estimate the prevalence of autism spectrum disorder in a representative school sample of Tarragona, Spain. Screening was performed through parents (N = 3727) and teachers (N = 6894), and 781 children were individually assessed. The overall estimated prevalence was 1.53% (1.78% in preschoolers; 1.30% in primary school children), being significantly higher than the 0.83% previously registered (0.92% and 0.74%, respectively). Respectively, 4.23% and 2.85% of the children showed subclinical autism spectrum disorder. Girls showed a significantly lower prevalence in all the conditions. Severity profiles were distributed as 46% mild, 47% moderate and 7% severe. A high ratio of males (90%) and children from Eastern Europe (16%) was found among severe autism spectrum disorder. Language therapy (51%) and psychological (65%) and educational supports (65%) were given to children with autism spectrum disorder. Pharmacological treatment was only found among school-aged children (37.5%). Public schools provided more educational support (72%) than private schools (36%). The heterogeneity of autism spectrum disorder makes it difficult to determine specific associated sociodemographic factors. The results confirmed a high prevalence of autism spectrum disorder in this province, suggesting a current under-diagnosis by public health services.