Objective: This study qualitatively explored barriers and facilitators of adherence to an online psychological intervention for cancer-related distress. Methods: Semi-structured interviews were conducted with 13 adults with cancer, randomised to receive either a 6-week intervention (n = 8) or attention control (n = 5) as part of a larger RCT. Transcripts were coded for themes and subthemes, and recruitment ceased when saturation of themes occurred. Results: Adherence overall was high: six participants completed all six modules, three completed five modules, two completed four modules, one completed one module, and one did not access the program. The total numbers of barriers (n = 19) and facilitators (n = 17) identified were equivalent and were categorised into five overarching themes: illness factors, psychological factors, personal factors, intervention factors and computer factors. However, the prevalence with which themes were discussed differed: illness factors (specifically cancer treatment side effects) were the main reported barrier to adherence; intervention factors (email reminders, program satisfaction, ease of use, program content) were the most common facilitators. Conclusion: While some factors were cited as both facilitating and barring adherence, and therefore reflective of personal preferences and circumstances, a number of recommendations were derived regarding (i) the best timing for online interventions and (ii) the need for multi-platform programs.