Background: Better parental knowledge about developmental milestones of children is linked to better parent–child interaction. Only a handful of surveys have been conducted to evaluate parental knowledge of developme ntal milestones in Arabic-speaking countries. Materials and Methods: The study is a cross-sectional, questionnaire-based observational investigation using a simple random sampling scheme. The sampling frame consisted of all parents attending the family medicine services during the period of conduct of the study. Poisson regression modelling techniques were used to analyse the adjusted effect of sociodemographic factors on knowledge score. Results: Three hundred and seventy-five parents agreed to be included in the study. Only 29 (7.7%) had an excellent knowledge level. Poor knowledge was found among 141 (37.6%) participants. Also, 180 (48%) participants were found to have an acceptable level of knowledge. Mothers achieved acceptable knowledge of developmental milestones better than fathers, and better knowledge was found in less-crowded houses. Physical developmental milestones were known by 304 (81.1%) participants, followed by cognitive development milestones ( n = 78, 20.8%), social development milestones ( n = 47, 12.5%) and emotional development milestones ( n = 85, 22.7%). The internet was the most used source ( n = 83, 22.1%). The most avoided sources were educational workshops ( n = 230, 61.3%), followed by doctors ( n = 159, 42.4%) and social media ( n = 130, 34.7%). Conclusions: We confirmed in this work that parental knowledge of children’s developmental milestones is suboptimum among Saudi parents. Use of the internet and avoidance of healthcare professionals is a source of concern. Care is required for children coming from overcrowded homes in terms of their developmental progress. Focus should be on educating parents about the social and emotional developmental milestones.