Pediatric obesity is a serious health condition associated with lower psychosocial adjustment in children/adolescents and their families. Mothers of children/adolescents with obesity usually present higher levels of parenting stress, and investigation shows that the authoritarian and permissive parenting styles are more used by those mothers. However, little is known about the influence of parental variables on children/adolescent outcomes. This study intended to: (1) investigate differences in the quality of life (QoL) between children/adolescents with obesity and healthy weight, (2) investigate differences between mothers regarding parenting stress and parenting styles, and (3) assess the mediating role of parenting styles in the relationship between parenting stress and the QoL of children/adolescents with obesity. The sample comprised 223 children/adolescents and their mothers (125 children/adolescents with a healthy weight and 98 with obesity). Children/adolescents completed measures of QoL (KIDSCREEN-10) and mothers completed measures of parenting stress (Parenting Stress Index) and parenting styles (Parenting Styles and Dimensions Questionnaire). Mothers of children/adolescents with obesity presented higher levels of parenting stress than mothers of children/adolescents with healthy weight. Mothers of adolescents with obesity used more a permissive parenting style than mothers of adolescents with healthy weight. Higher levels of parenting stress were associated with poorer QoL in children/adolescents through the use of permissive parenting style. This study emphasizes the connection between parenting stress, parenting styles and children’s/adolescent’s QoL, providing innovative input to the field by identifying mechanisms that might account for the link between family and child variables.