<h3>Purpose/Objective(s)</h3> Equitable inclusion of racial and ethnic participation in clinical trials is crucial to improving disparities in healthcare, especially for historically marginalized populations. However, a recent review noted a prevalence in underreporting of racial and ethnic demographics in published studies, with Hispanic and Black representation recorded in 10% and 38.2% of cancer drug clinical trials and representing 3.1% and 6.1% of trial participants. Proton therapy relies on clinical investigations to guide treatment protocols; yet, few publications have investigated their demographic composition. We aimed to describe the racial and ethnic demographics of patients enrolled on published phase II U.S. clinical trials involving proton therapy. <h3>Materials/Methods</h3> Published manuscripts were identified in PubMed, Embase, World of Science, and Cochrane. Phase II trials evaluating proton therapy for patients in the United States were included. For each article included, data were collected on authors, title, and publication year; clinical trial numbers were verified. Additional data included tumor site, primary institution, sample size, reported race/ethnicity data, and raw number/percentile of race/ethnicity. Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines were used for reporting. <h3>Results</h3> Overall, 970 titles were identified; 636 remained after duplicate screening; and 75 full-text articles were assessed. Ultimately, we identified 38 eligible manuscripts for inclusion with a total 2,648 patients. Only 15 (39%) of the publications reported race/ethnicity. Of these 38, 8 (21%) and 10 (26%) documented Hispanic or Black representation of trial participants, respectively. Moreover, 6 (16%) documented both Hispanic and Black representation in trial participants. Of 1,409 patients with documented race/ethnicity, 89.0% (n=1,254) were non-Hispanic white, 5.3% (n=75) were Black, and 2.2% (n=31) were Hispanic. Other and unknown race/ethnicity comprised the remaining patients (3.5%; n=49). <h3>Conclusion</h3> In this analysis, we identified underreporting of demographic data in published phase II proton therapy trials, unfortunately mirroring underreporting for cancer drug clinical trials. We also note dramatic Black and Hispanic underrepresentation across the trials in which race and ethnicity are reported. Findings highlight the urgent need to identify and address barriers to proton therapy trials for Black and Hispanic patients so that clinical trials in radiation oncology are representative of patients seen in clinical practice.