Background <p>Frequent HIV testing among people at high risk of HIV infection is critical for early diagnosis and prompt antiretroviral therapy (ART) initiation for those who test HIV positive, as well as provision of effective biomedical HIV prevention modalities like pre-exposure prophylaxis (PrEP) for those who test negative. Despite World Health Organization (WHO) guidelines recommending retesting for HIV every 3–6&#xa0;months for adults at increased risk<b>,</b> regular retesting is infrequent in sub-Saharan Africa. Large-scale, multi-arm field experiments (“megastudies”) can evaluate many different behavioral interventions among the same population and at the same time rapidly identify behavioral solutions and overcome the challenge of comparing results across standalone trials. We describe the protocol for a megastudy that will simultaneously test multiple low-cost behavioral interventions to increase HIV retesting among adults at high risk of HIV infection in rural Kenya and Uganda.</p> Methods <p>We will conduct a 12-arm randomized trial of light-touch (“nudge”) behavioral interventions among 30,000–40,000 HIV-uninfected adults aged ≥ 15&#xa0;years in western Kenya and southwestern Uganda who self-report increased risk of HIV infection and have access to a mobile phone. Following a baseline HIV-negative test at a high-volume HIV testing center, participants will be individually randomized in parallel to one of 12 arms, including 10 different intervention arms comprising low-cost messages that serve as nudges for retesting, an intervention arm with a financial incentive for retesting, or a control arm with standard of care post-test counseling services. The primary outcome will be HIV retesting at any study venue 3–6&#xa0;months after the initial baseline HIV test. We will also measure HIV-positivity, linkage to care and ART initiation, and PrEP referral acceptance at follow-up. </p> Discussion <p>This study will provide new information on low-cost interventions to promote HIV retesting for adults at elevated risk of infection in East Africa. The methods and results of this study will provide valuable insights for those engaged in HIV service delivery and influence how researchers and programs design and test low-cost, scalable interventions to promote health behaviors. </p> Trial registration <p>ClinicalTrials.gov NCT06971367. Registered on May 6, 2025, <a href="https://clinicaltrials.gov/study/NCT06971367">https://clinicaltrials.gov/study/NCT06971367</a></p>