Parental reflective functioning refers to parents' capacity to consider their child's internal experiences and is associated with secure parent-child attachment, sensitive parenting behavior, and positive child socioemotional development. However, research into determinants of parental reflective functioning in large diverse samples has been scarce. Therefore, using a large multinational sample and longitudinal design, we aimed to: (a) identify sociodemographic determinants of parental reflective functioning; (b) investigate whether parental emotion regulation is a psychological determinant of parental reflective functioning; and (c) assess whether child negative affect, parent stress, and child age moderate longitudinal associations between parent emotion regulation and parental reflective functioning. Data were two time points of the Child and Parent Emotion Study following 2,208 parents (68% female) of a child aged 0-10 years residing in English-speaking countries. Parent emotion dysregulation, parent male cisgender status, speaking a language other than English, younger parent age, and older child age were associated with lower parental reflective functioning 12 months later, as indicated by higher levels of prementalizing. Child negative affect moderated the association between parent emotion dysregulation and parental reflective functioning, whereas child age and parent stress did not. Specifically, parent difficulty controlling impulses when distressed was associated with higher levels of prementalizing modes of reflective functioning only when child negative affect was moderate-to-high. The interplay between parent emotion dysregulation and child negative affect highlights the importance of considering both parent and child characteristics in intervention planning and suggests that personalizing parenting support according to these characteristics may be beneficial. (PsycInfo Database Record (c) 2023 APA, all rights reserved).