Background More than 50% of all patients with breast cancer experience fatigue symptoms during and after their treatment course. Recent evidence has shown that fatigue is partly driven by cognitive biases such as the self-as-fatigued identity bias, which may be corrected with computer-based cognitive bias modification (CBM) techniques. Objective The aim of this study was to design a CBM-training app by adopting a cocreation approach. Methods Semistructured interviews were conducted with 7 health care professionals, 3 patients with breast cancer, and 2 patient advocates. The aim of the interviews was to collect input for the design of the CBM training, taking the values and preferences of the stakeholders into account, and to determine the timing and implementation of the training in the treatment course. Results Overall, the interviews showed that the concept of CBM was accepted among all stakeholders. Important requirements were revealed such as the training needs to be simple and undemanding, yet engaging and persuasive. Based on the results, an eHealth app IVY (Implicit VitalitY) was created. The findings from the interviews suggested that IVY should be offered early in the breast cancer treatment course and should be carefully aligned with clinical treatment. Conclusions The findings of this study show that using CBM as a preventive approach to target cancer-related fatigue is an innovative technique, and this approach was embraced by breast cancer stakeholders. Our study suggests that CBM training has several benefits such as being easy to use and potentially increasing perceived self-control in patients.