Objective: Cancer Clinical Nurse Specialists (CNSs) play a key role in PA promotion in cancer care in the United Kingdom and patients value PA support from their assigned CNS. Smartphone applications (apps) can be an effective way to increase PA but little is known about CNSs' opinions about the use of apps to promote PA among people affected by cancer. This study aimed to assess CNSs' opinions on the CNS role in PA promotion and the potential of smartphone-based PA interventions in cancer care. Methods: Breast, prostate and colorectal cancer CNSs were recruited via advertisements distributed by professional organisations. In-depth semi-structured telephone interviews were conducted and analysed using thematic analysis. Results: 19 participants took part; 9 (47%) were colorectal cancer CNSs, 6 (32%) were prostate cancer CNSs and 4 (21%) were breast cancer CNSs. CNSs acknowledge their role in discussing and supporting PA among their patients but recognise that they sit within a wider system necessary for effective PA promotion among people affected by cancer. They highlighted the influence that the national/policylevel changes in survivorship care in the UK has had on their ability to bring PA into consultations and that this has helped to raise awareness about the benefits of PA after cancer (e.g. on fatigue, quality of life, risk of cancer recurrence). However, CNSs felt they lack specific knowledge and confidence about what exactly to recommend or how to encourage patients who might require greater levels of support to increase PA (e.g. patients with advanced disease, several comorbid conditions or low levels of motivation/interest to increase PA). CNSs recognise the benefits that app-based PA interventions offer and feel that this could be an effective way to support patients who want to engage with PA support in this way but that it may not be of interest or suitable for all of their patients. CNSs discussed factors that could influence effective implemention of app-based PA support in routine care. Conclusions: The results of this study can inform the development of an app-based PA intervention for people affected by cancer that could be implemented into routine cancer care.