Objective: Despite its importance for mental and physical health, many adults fail to meet current physical activity recommendations. Furthermore, most adults who begin a physical activity programme revert to being less active or even inactive within the first 6 months. Fitness bootcamps represent a potential intervention for improving physical activity in healthy adults. However, no study to date has examined the influences on long-term participation in fitness bootcamps, which is the first step to developing an effective intervention. Using the Behaviour Change Wheel (BCW) framework, this study identified the facilitators and barriers to long-term (1 year or more) fitness bootcamp participation, which were then linked to behaviour change techniques (BCTs) to facilitate intervention development. Design: Qualitative research design. Setting: Data collection occurred in England. Method: Interviews were conducted with 15 long-term fitness bootcamp participants. Results: Thematic analysis revealed 17 facilitators and 6 barriers to long-term fitness bootcamp participation. Participants highlighted the importance of facilitators such as convenience, being outdoors and enjoyment. The social environment, including having a positive instructor and supportive group members, was further noted as a key influence on participation. While long-term participants faced occasional barriers to their routine such as physical injury, bad weather and competing events, the impact of these could be minimised through appropriate BCTs such as 'demonstration of the behaviour' and 'restructuring the social/physical environment'. Conclusion: Findings from this study highlight the importance of social and environmental factors to promoting long-term fitness bootcamp participation and the inclusion of social and physical environmental restructuring as key intervention components.