Background <p>Diversity of study populations in clinical trials is essential to addressing health outcome disparities in cancer care. The objective of this study was to examine the inclusion of minority race/ethnicity groups in breast cancer clinical trials published in the United States of America (USA) 2012–2022.</p> Patients and Methods <p>Breast cancer clinical trials published as a full manuscript in the USA 2012–2022 were identified. Trial characteristics and race/ethnicity of subjects were collected to assess the degree of reporting and inclusion of minority populations. Participation to Prevalence Ratios (PPRs) were utilized as a measure of inclusion.</p> Results <p>In 1068 trials (<i>n</i> = 871,683 subjects), race/ethnicity was reported in 785 (73.5%), increasing from 2012 to 2016 (68.8%) to 2017 to 2022 (79.3%, <i>p</i> &lt; 0.001). Higher rates of race/ethnicity reporting were independently associated with later publication year, randomized or phase II trials, and those reporting social determinants of health. Of the 732,969 subjects for whom race/ethnicity was reported, 16,551 (2.3%) were Asian American/Pacific Islander (Asian), 43,288 (5.9%) were Black/African American, 22,464 (3.1%) were Hispanic/Latinx, and 570,897 (77.9%) were Non-Hispanic White. A PPR above 0.8 was achieved in 27.3% of trials that reported race for Asian subjects, 44.7% for Black/African American subjects, 27.4% for Hispanic/Latinx subjects, and in 5.7% for all three minority groups. All four primary race groups in the USA were represented in 24.7% of trials, and race/ethnicity was included as a factor in outcome analysis in 8.7%.</p> Conclusions <p>Although race/ethnicity reporting has improved, minority populations remain gravely underrepresented in breast cancer clinical trials, limiting the generalizability of trial results in these populations.</p>