Parental reflective functioning (PRF) is considered to be a key factor in the development of emotion regulation in children, which in turn plays an important role in their psychosocial functioning. This study aimed to explore the mediating role of children's emotion regulation in the relationship between parental reflective functioning and children's social, emotional, and behavioural functioning in a sample of preschool-age children. Participants were 257 mothers (Mean age = 35.63, SD = 4.55) and their preschool children (Mean age = 4.83, SD = 0.89; 46.7% girls), recruited through convenience sampling. Mothers' reflective functioning was measured using Parental Reflective Functioning Questionnaire (PRFQ), which includes three subscales: Pre-Mentalizing (PM), Certainty about Mental States (CMS), and Interest and Curiosity in the Mental States (IC). Also, mothers reported on their child's emotion regulation and psychosocial functioning using the Emotion Regulation Checklist (ERC), Strengths and Difficulties Questionnaire (SDQ), and Social Skills Rating System (SSRS). Findings revealed that PM, as expected, was positively associated with psychosocial difficulties and negatively associated with emotion regulation and social strengths. IC, as expected, was negatively associated with psychosocial difficulties and positively associated with emotion regulation and social strengths. CMS, unexpectedly, was not associated with psychosocial difficulties and was positively associated with emotion regulation and social strengths. In addition, results of structural equation modelling showed that children's emotion regulation mediated the relationship between parental reflective functioning and children's social, emotional, and behavioural functioning. Our results highlight the importance of parental reflective functioning and children's emotion regulation for children's social, emotional, and behavioural functioning, and shed light on the possible role of emotion regulation through which parental reflective functioning exerts its influence on children's psychosocial functioning.