Parental reflective functioning – parents’ capacity to envision the mental states underlying their child’s behavior – plays an important role in parenting behavior, parental well-being, and children’s psychosocial outcomes. Most studies have examined parental reflective functioning in terms of relatively stable interindividual differences between parents. This is unfortunate because theoretical accounts suggest that this capacity is susceptible to intraindividual fluctuations. Parenting stress, in particular that associated with difficult child behavior, has been described as a factor that can put parental reflective functioning under pressure. Using a multilevel approach, this 7-day diary study investigated day-to-day fluctuations in parental reflective functioning and its associations with daily parenting stress and perceived internalizing and externalizing adolescent difficulties. Parents of community adolescents (N = 128) and adopted adolescents (N = 28) were sampled because adoptive parents face unique stressors that may challenge their reflective capacities. Results indicated that daily parenting stress was associated with more daily prementalizing (i.e., severely biased mentalizing), less daily certainty about mental states, and less interest and curiosity in the adolescent’s mental states. Whereas externalizing difficulties were similarly related to more daily prementalizing and less certainty about mental states, findings for internalizing difficulties were mixed. Most associations were consistent across biological and adoptive parents.