Background: The increased tasks and responsibilities involved in supporting a parent with dementia (PWD) can induce distress in adult-child caregivers. Previous studies have shown that distress can be influenced by PWD and caregiver determinants, but few studies have considered the associations between these variables. Objective: This study tested a complex model of adult-child caregiver distress in which PWD and caregiver determinants and their associations are considered. Methods: 159 adult-child caregivers participated in this online study. PWD and caregiver determinants were assessed using questionnaires and their associations were investigated using the partial least squares path method. Results: The model showed a significant partial mediation through self-efficacy (confidence in one’s ability to organize and manage caregiving situations) between poor self-rated health and distress. Self-efficacy was a significant mediator of the relationship between informal social support and distress, and between preparedness and distress. The direct path between parental overprotection and distress was significant. The association between care and distress was significantly stronger for adult-child caregivers not living with their PWD. Conclusion: The model revealed the important mediating role of self-efficacy. Clinical interventions should improve the preparedness of adult-child caregivers and the quality of social support. The positive perception of their self-rated health may thus be promoted.