Underrepresented populations in dementia prevention research, including minoritized racial/ethnic groups, individuals with lower socioeconomic status, and others facing social and structural disadvantages, are disproportionately affected by dementia risk. This scoping review examined barriers and facilitators to recruitment, engagement, and retention of these populations in Alzheimer’s disease and related dementias (ADRD) prevention studies, synthesizing evidence from both empirical studies and review articles. Guided by PRISMA-ScR and the conceptual structure described by Gilmore-Bykovskyi et al., findings were synthesized from 19 reviews and 53 empirical studies. Findings were interpreted with attention to how overlapping factors—such as ethnicity, age, gender, and structural inequities—may influence study participation. Studies originated primarily from the United States (U.S.). Five key themes were identified: 1) mistrust, 2) stigma and limited research literacy, 3) logistical and financial constraints, 4) communication gaps and lack of team diversity, and 5) systemic-level barriers. Facilitators included culturally tailored outreach, long-term community partnerships, and inclusive study design. Retention strategies remain underreported, and little is known about the non-U.S. context. These findings highlight the need for context-specific, multi-level strategies that address the intersecting barriers faced by underrepresented groups to support equitable participation in dementia prevention research, and ultimately, dementia prevention.