<p dir="ltr">A growing appreciation of trauma and its implications on the health of those affected has led to the adoption of trauma-informed principles across different sectors where service providers interact with those who have experienced trauma. Those in the physical activity sector have begun to incorporate trauma-informed principles into programme design and delivery. However, there remains no consensus on how trauma-informed principles are operationalised and implemented within a physical activity context. The present study aims to explore what it means to be trauma-informed in physical activity, identifying the principles essential for physical activity programmes. Semi-structured interviews were conducted with 17 participants (10 female, 7 male) from organisations facilitating and delivering trauma-informed physical activity programmes. A critical realist-informed approach to thematic analysis was used to analyse the data. Four themes were developed: (i) approaching with considerate curiosity, (ii) boundaries and scope of practice, (iii) the importance of shared lived experience, and (iv) meeting people where they are. These themes highlight principles that must be present in trauma-informed physical activity, such as physical activity providers understanding their limits of practice and referring to other services when these limits are met. They also outline the importance of understanding and empathy in trauma-informed physical activity. For trauma-informed physical activity programmes to be effective, these principles must be consistent across the programmes’ design and delivery. This study progresses understanding of trauma-informed principles and offers professionals in the physical activity sector insights of how they can ensure their programmes are appropriate for those who have experienced trauma.</p>