Abstract <p>Endometrial cancer survivors spend over half the day sedentary (SB), and prior interventions to promote physical activity (PA) have had limited success. Interventions targeting SB reduction are needed but must incorporate behavior change techniques for effectiveness and sustainability.</p> Purpose <p>To use intervention mapping refined by patient advocates to develop a theory- and evidence-guided intervention to reduce SB in endometrial cancer survivors.</p> Methods <p>A six-step intervention mapping protocol was applied: (a) needs assessment, (b) identification of behavioral impacts and intended behavior modifications, (c) linking determinants of outcomes with behavior change techniques, (d) development of the intervention, (e) formulation of an implementation plan, and (f) development of an evaluation plan. Refinement occurred through one-on-one interviews with patient advocates to identify facilitators, preferences, and barriers.</p> Results <p>The proposed multi-study sequential Sitting Time Elimination Program (STEP) after Endometrial Cancer targets SB reduction and PA promotion in a stepwise manner. Patient advocate input refined the intervention, which incorporates self-monitoring with wearables, small group health coaching, and a smartphone app to support engagement.</p> Conclusions <p>Patient advocates identified more facilitators than barriers to the proposed intervention. Adaptations addressing barriers have been incorporated, and the intervention is prepared for feasibility and effectiveness testing.</p> Implications for Cancer Survivors <p>Incorporating patient-informed, multi-component interventions like STEP may help achieve sustainable reductions in SB and increased PA, addressing gaps where prior interventions have fallen short for endometrial cancer survivors.</p>