Parental reflective functioning (PRF) refers to parents’ capacity to reflect on mental states such as their own and their child’s feelings, thoughts, and intentions. Studies suggest that PRF is an important factor in parental behavior; however, only a few studies have examined the effect of early PRF on infant socioemotional development. The Parental Reflective Functioning Questionnaire (PRFQ) was developed as a brief, multidimensional measure to assess PRF. Recently, a modified version, the PRFQ-I, was suggested as a more accurate measure of PRF in an at-risk sample of mothers of infants. The study aims were to examine the factor structure and factorial invariance across parental gender of the PRFQ-I in a community sample, to compare PRF dimensions across mothers and fathers, and to investigate the association between PRF dimensions and infant socioemotional development. We examined this within a longitudinal design in a community sample of 1243 first-time mothers and 512 fathers. The results supported the expected factor structure of PRFQ-I and invariance across parental gender. Significant differences between mothers and fathers were found on the three PRF dimensions comprised by the questionnaire. The PRF dimensions of pre-mentalizing and certainty about mental states significantly predicted infant socioemotional development. Findings show that the PRFQ-I is a valid measure in parents of infants and suggest that PRF in early parenthood is an important factor in infant socioemotional development. Identification of first-time parents with limited PRF enables early intervention and may thus prevent limited PRF from having potentially adverse effects on infant socioemotional development.