Background: A high percentage of patients with cancer experience cognitive impairment after cancer treatment, resulting in a decreased health-related quality of life and difficulty returning to work. Consequently, there is a need for effective treatment options to improve cognitive functioning in these patients. In a healthy aging population, multidomain web-based lifestyle interventions have been found to be effective in preventing cognitive decline and improving cognitive functioning. Objective: This study aims to investigate the feasibility and effectiveness of the web-based lifestyle intervention Mijn Fitte Brein (My Fit Brain [MFB]) on cognitive functioning in patients with cancer returning to work. Methods: The study consists of a feasibility study (N=10), followed by a randomized controlled trial (RCT; N=220). Patients will be recruited by their occupational physicians after their return to work following cancer treatment. Mijn Fitte Brein is organized into 4-week cycles in which patients set a lifestyle goal using the Goal Attainment Scale, receive weekly tips and support, and finally evaluate whether they succeeded in achieving this goal. Lifestyle goals are based on 6 domains: physical exercise, diet, sleep, stress, alcohol use, and smoking. In the feasibility study, data on user experience (structured interview) and usability, assessed with the Post-Study System Usability Scale, will be collected and used to optimize Mijn Fitte Brein. In the RCT, patients will be randomized 1:1 between an intervention group and a control group. Patients will be assessed at baseline, 3 months, and 6 months. The primary outcome measure is subjective cognitive functioning, assessed with the Functional Assessment of Cancer Therapy-Cognitive Function (FACT-Cog). Secondary outcome measures are lifestyle, objective cognitive functioning, and work and psychosocial factors. Results: Recruitment for the feasibility study has started in February 2020. As of July 2020, however, no patients have been enrolled (due to COVID-19 restrictions). The findings of the feasibility study will be used to optimize the Mijn Fitte Brein intervention. Enrollment for the RCT will continue when possible. The feasibility study will take 6 months (including making adjustments to the intervention), and the RCT will take 2 years. The final results are expected in 2024. The results of the feasibility study and the RCT will be published in peer-reviewed journals. Conclusions: This is the first time the feasibility and efficacy of a multidomain web-based lifestyle intervention will be studied in patients with cancer. If Mijn Fitte Brein is found to be effective in decreasing cognitive complaints in these patients returning to work, it will be a promising treatment option because of being both affordable and accessible.