Background: Health behaviors and motives within family units are likely to be associated with the motives and behaviors of other family members. A potential mechanism for these relations is that parents citing autonomous motives toward physical activity are more likely to support their child’s autonomous motivation to be active. The current study tested a model specifying relations between parent autonomous motivation and parent, child, and parent-and-child joint physical activity behavior with parent autonomy-supportive parenting, child-perceived autonomy support, and child autonomous motivation specified as mediators of the autonomous motivation-child physical activity relationship. Method: Data on autonomous motivation, autonomy-supportive parenting, perceived autonomy support, and physical activity, both alone and jointly between parents and children, were collected from a sample of 88 Finnish parent–child dyads and analyzed using a partial least squares structural equation modeling. Results: Findings indicated associations between parent autonomous motivation and parent and joint parent–child physical activity, and an indirect effect of autonomy-supportive parenting on child physical activity mediated by child-perceived autonomy support and autonomous motivation. Conclusion: Results provide a potential explanation for relations between parental motives and child physical activity as a function of supportive parenting and signpost potential targets for family-based behavior change interventions.