New estimates indicate that many children in low- and middle-income countries are at risk of developmental delay (DD). This study aimed to assess the prevalence and socioeconomic determinants of undetected DD, and mean developmental scores (MDS) in preschool-aged children in the northwest of Iran. This study was conducted on 615 children aged 36-60 months. For the early detection of DD, the Persian version of the Ages and Stages Questionnaires (ASQ) was used for face-to-face interviews with mothers. Our findings showed that the prevalence of DD was 13.98% (1.63% in communication, 2.28% in gross motor skills, 3.41% in fine motor, 2.67% in problem-solving and 3.58% in social-personal domains). Multivariate analyses showed that DD was higher in preterm children, single-child families, and the second SES quantile. Due to the high prevalence of DD and the importance of early identification and intervention, it is necessary to diagnose DD at early ages.