Purpose <p> Internet-delivered cognitive behavioral therapies and lifestyle interventions are promising for reducing psychiatric symptoms in cancer survivors, but little is known about the contribution of specific treatment components to observed effects. A randomized factorial trial could allow for evaluation of the unique contribution of treatment components to overall treatment effects. The IN-FACT-0 trial assessed the feasibility of such an approach for building an online support program for reducing long-term psychiatric symptoms in cancer survivors.</p> Methods <p>In a full factorial experiment, 40 survivors of breast, testicular, or thyroid cancer with clinically significant depressive symptoms, anxiety, or fear of recurrence were randomized to combinations of three treatment components: (i) behavioral activation, (ii) systematic exposure, and (iii) healthy lifestyle promotion. All groups, including a control group, received rudimentary support and kept a symptom diary.</p> Results <p>On average, therapists rated the feasibility of the study design 9 on a scale from 0 (“not feasible at all”) to 10 (“fully feasible”). The Credibility/Expectancy Scale mean was 36.6 (SD = 8.2). Participants initiated a mean of 6.1 (SD = 2.2, median = 7) out of 8 modules, and 90% (36/40) completed the post-treatment assessment. The mean Client Satisfaction Questionnaire score was 24.3 (SD = 5.7). One adverse event each was reported by 2/36 (6%) participants. No event was serious. The mean reduction was large in depressive symptoms (<i>d</i> = 0.83) and moderate in general anxiety (<i>d</i> = 0.55) and fear of recurrence (<i>d</i> = 0.54).</p> Conclusions <p>This randomized factorial trial design was feasible.</p> Implications for Cancer Survivors <p>This trial design may be used to develop effective, evidence-based interventions for cancer survivors with psychiatric symptoms.</p> Trial registration <p>ClinicalTrials.gov NCT06046573 (registered 2023–09-21).</p>