Diet-related health inequalities are a persistent public health challenge in high-income countries, disproportionately affecting socially and economically disadvantaged populations. This study aims to map the existing evidence on diet-related health inequalities in high-income countries through a scoping review of observational studies, identifying populations most affected and key dietary outcomes across social determinants of health. We conducted a systematic search of MEDLINE, Web of Science, Scopus, and Embase for observational studies published between January 2011 and March 2021. Eligible studies assessed diet-related health outcomes stratified by ≥1 Place of Residence, Race/Ethnicity, Occupation, Gender/Sex, Religion, Education, Socioeconomic Status, and Social Capital (PROGRESS)-Plus determinant. We followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews guidelines and registered the review with International Prospective Register of Systematic Reviews (CRD42021234567). Data were charted and analyzed thematically according to PROGRESS categories. A total of 163 studies were included. Most studies focused on education, socioeconomic status, and place of residence, whereas fewer addressed gender identity, sexual orientation, or disability. Common dietary indicators included fruit and vegetable intake, dietary patterns, and food group consumption. Evidence consistently showed that lower education and income levels were associated with poorer dietary outcomes. Notably, certain population groups (for example, ethnic minorities, rural residents, and individuals with low education or income) experienced cumulative disadvantages. The scoping review highlights persistent and intersecting diet-related health inequalities in high-income countries. It underscores the need for standardized indicators and intersectional approaches in monitoring, research, and policy making.