Wounded, injured, and/or sick (WIS) military veterans face significant physical and psychosocial challenges following discharge from service. Physical activity can have many positive effects on the holistic wellbeing of such individuals. However, little knowledge exists regarding the perceived barriers and benefits of physical activity within this population, creating challenges surrounding physical activity promotion. Therefore, this study was designed to identify key barriers and benefits among this population, so that informed approaches to encourage participation in physical activity can be developed. A questionnaire related to the perceived barriers and benefits of physical activity was completed by 105 WIS British military veterans. Participants were predominantly male, physically active, served in the British Army, and described their injury as frequently impacting their daily living. Factor analysis revealed that poor mental health, negative beliefs about physical activity, and low beliefs about physical capability were prominent barriers represented in the data. However, only veterans’ beliefs about their physical capability were related to physical activity levels and differed between active and insufficiently active participants. Beliefs identified as benefits of physical activity that correlated with physical activity levels were improved mental and physical health, a sense of purpose, and increased physical fitness. Subsequently, barriers and benefits were categorized using the Behaviour Change Wheel, a behavior change framework, that indicated intervention functions of education, incentivization, and persuasion might be effective methods of increasing physical activity behavior among WIS veterans. Ultimately, this will lead to greater engagement with physical activity and improved health and wellbeing within this population.