Background: This study aimed to develop a reliable instrument, the Dogs and Physical Activity (DAPA) tool, for measuring important attributes and scales relating to the dog-walking behavior of dog owners. Methods: Items measuring dog-specific individual, social environmental, physical environmental, and policy-related factors that affect dog owners’ walking with their dogs were assessed for test–retest reliability. Factor analysis was undertaken to demonstrate that the collection of test items had underlying constructs consistent with the theoretical framework. Results: DAPA-tool items had test–retest reliability scores >.7, indicating a high level of stability. Distinct general and dog-specific constructs of subscales measuring dog-supportive features of parks, barriers to dog walking, and behavioral beliefs about the outcomes of regular dog walking were demonstrated through factor analysis. Conclusions: The DAPA tool is the first comprehensive, reliable tool for measuring important attributes and scales relating to dog owners’ physical activity and the context-specific factors that affect owners’ walking with their dogs.