Background: The purpose of the current study was to examine association between physical activity (PA) of parents of children with developmental disabilities and theory of planned behaviour-related variables. Methods: A total of 95 participants completed questionnaires that measured the following variables: PA behaviours, action planning, intention, affective attitude, instrumental attitude. A path analysis was used to examine the associations between the variables. Results: Among participants, 40% did not meet the PA guideline. PA affective attitude was positively associated with PA intention (ß = 0.30, p = 0.009) and PA action planning ([beta] = 0.22, p = 0.03). In turn, both PA intention and PA action planning were associated with moderate-to-vigorous PA in parents of children with developmental disabilities ([beta] = 0.23, p = 0.008; [beta] = 0.52, p < 0.001, respectively). Conclusion: Physical activity in parents of children with developmental disabilities were appropriately explained by theory of planned behaviour-related variables.