Healthcare discrimination experience is expected to be a consequential variable that explains racial/ethnic inequities in patient activation and health; however, existing scales assessing healthcare discrimination experience are limited by insufficient psychometric development and overly narrow construct definitions. A new Healthcare Discrimination Experience Scale was developed, validated, compared to an existing scale, and used to estimate effects in explaining racial/ethnic health inequities. Across two studies, 975 patients with hypertension or diabetes (43% Black, 10% other Persons of Color, 47% White, 53% having household incomes < 40 thousand dollars) were recruited through marketing research panels to complete online questionnaires. Compared to an existing measure, the new scale better detected differences between People of Color and White people. It produced good results in confirmatory factor analysis and item response theory analysis, and it mediated the effects of racial/ethnic identity on eight variables regarding patient-practitioner relationships, treatment adherence, general health, blood pressure, and life stress. The new scale is valid for assessing a broadly defined healthcare discrimination experience construct in diverse patients with chronic medical conditions, and it is more sensitive to group differences than the best existing alternative scale. Compared to research using unvalidated scales, the results of this study demonstrate that healthcare discrimination experience plays a larger role in explaining racial/ethnic inequities in patient activation and health.