Background: Despite the well-established physical and psychological benefits of physical activity (PA) for cystic fibrosis (CF) populations, adherence to PA programmes within this population remains low. This study aimed to investigate the factors that influence engagement in physical activity, and to explore exercise preferences, among adults with cystic fibrosis. Methods: Semi-structured telephone interviews were conducted. Participants were twenty-one adults (mean age 35 years, SD ± 8) with an established diagnosis of CF, living in Ireland. Interview scripts were digitally recorded and transcribed verbatim. Thematic analysis was used to analyze the data. Results: Four main themes emerged: barriers, motives, value of exercise-related outcomes, and exercise preferences. The main barriers included: fatigue, low energy levels, time, the weather, and exercise-related confidence. Motives for regular physical activity participation included: enjoyment and perceived competence. Participants who were regularly active valued personally identified exercise-related outcomes such as, accomplishment and affect regulation. Participants indicated a preference for home-based physical activity programs compared to gym- or facility-based programs.Conclusion: Interventions aimed at promoting physical activity among adults with CF should involve home-based programs that foster autonomous motivation confidence, enjoyable activities, personally identified outcomes and competence. Clinical Implications: To increase physical activity participation among adults with CF, home-based interventions that pay attention to the patients’ personally-valued exercise outcomes may be required. Wearable technology that provides real-time feedback may enhance feelings of competence and accomplishment, found to underpin autonomous motivation.