People with spinal cord injuries (SCI) are physically inactive. Smartphone applications (or apps) may prove as one strategy to overcome this. This study examines the theoretical underpinning of a novel mHealth intervention that aims to improve physical activity in people with SCI, namely, the Accessercise smartphone app, using the behaviour change wheel (BCW), Accessercise was evaluated using the BCW in eight steps across the following three stages: (I) understanding the behaviour, (II) identifying intervention options, and (III) identifying content and implementation options. Thirteen target behaviours were identified to improve physical activity and reduce sedentary behaviours in adults with SCI, including goal setting and monitoring, increasing self-confidence, interest and motivation for undertaking physical activity, improving the knowledge/awareness of available physical activity opportunities and resources, and reducing stigma and negative attitudes associated with physical activity. Accessercise incorporates the necessary components for adults with SCI to be physically and psychologically capable of undertaking physical activity, offering social and physical opportunities to reduce sedentary behaviours, and supports automatic and reflective motivation. This systematic approach of assessing the theoretical underpinning of Accessercise in the context of the BCW has revealed potential mechanisms of action for improving physical activity in adults with SCI. This serves as a blueprint to inform further intervention development, as well as high-quality effectiveness studies, namely, randomised controlled trials, assessing whether fitness apps can improve physical and psychological health outcomes in individuals with SCI.