Purpose: Understanding motivational factors related to physical activity (PA) maintenance is essential in promoting long-term exercise benefits. This study explored the impact of self-efficacy (SE) on post-intervention PA maintenance in patients with hip osteoarthritis. Method: An SE-theory based mixed-methods sub-study of a trial investigating the effects of 4 months supervised exercise in patients with hip osteoarthritis. Questionnaire data (n = 52; baseline and 12 months) on PA and SE (Arthritis Self-Efficacy Scale, ASES, score-range 10–100) were analysed (Mann–Whitney test) for differences in characteristics of maintainers and non-maintainers. Semi-structured individual interviews (n = 15; at 12-months follow-up) were analysed using directed content analysis. Results: Compared to non-maintainers (n = 9; 17%) maintainers (n = 31; 60%) had improved (p < 0.01) in median scores of ASES (Pain: +12 versus −32 points; Function: +7 versus −9 points; Other Symptoms: +11 versus −26 points) from baseline to 12 months. Experiences of possessing required skills, inspiration by other participants, encouragement from physical therapists and altered interpretations of PA-induced physiological conditions contributed to increased SE and PA maintenance. Moreover, experienced symptoms, exercise outcome expectations and obligation towards the study influenced maintenance. Conclusion: SE contributes to understanding of post-intervention PA maintenance in patients with hip osteoarthritis. However, disease-related factors and clinical trial participation appears significant too. •Implications for Rehabilitation •Patients’ perceived self-efficacy for physical activity contributes to the understanding of post-intervention physical activity maintenance in patients with hip osteoarthritis. •Practitioners may benefit from incorporating the self-efficacy theory in the planning and execution of exercise interventions to promote post-intervention physical activity maintenance and long term health benefits. •Post-intervention physical activity maintenance may be increased by focussing on the patients’ exercise self-efficacy through verbal persuasion and support, disease-specific information and information on normal physiological responses to exercise combined with an individualised training progression to support experiences of success and achievement of desired outcomes.