Children and youth with autism spectrum disorder engage in less physical activity than neurotypically developing peers. This may be due to factors associated with autism spectrum disorder at the individual and environmental level that can make physical activity participation more challenging. Parent support is a known determinant of physical activity among children and youth; however, limited research has explored the relationship between parent physical activity support behaviour and child physical activity behaviour within the autism spectrum disorder population. Guided by the multi-process action control framework, this study examined the relationship between parent physical activity support behaviour and physical activity levels of children and youth with autism spectrum disorder. Parents (n = 201) of school-aged children and youth with autism spectrum disorder completed measures of parent physical activity support (intentions, behavioural regulation, support behaviour), as well as their child's physical activity behaviour. Congruent with the multi-process action control model, intentions to provide physical activity support were significantly associated with parent physical activity support behaviour. Behavioural regulation of physical activity support mediated this relationship, which in turn significantly predicted child physical activity behaviour. Findings suggest parents play an instrumental role in the physical activity behaviour of children and youth with autism spectrum disorder. Family-level interventions targeting parents' behavioural regulation strategies to provide physical activity support may be an effective strategy to increase physical activity in children and youth with autism spectrum disorder.