Mental disorders affect many different groups around the world, and disadvantaged groups are often more severely affected. Neighbourhood green spaces (GS) can improve mental health, especially in disadvantaged groups. Many countries address social inequality and inequity through GS interventions. However, current evidence shows inconsistencies, which may result from the study site, research design, socio-demographically diverse samples, inclusivity considerations, and the different metrics used to quantify GS exposure and mental health benefits. Few conceptual models explain how neighbourhood greenery can act as a structural intervention. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method and retrieved 5559 documents from eight databases to examine whether neighbourhood GS can modify mental health associations in disadvantaged groups. We found that neighbourhood GS had substantial protective effects on the mental health of disadvantaged groups. However, disadvantaged people are more influenced by GS quality than by other GS exposures, such as GS usage, distance, and accessibility. Improvements in subjective well-being were most pronounced in terms of mental health outcomes. Mechanistically, neighbourhood GS improves mental health mainly through increased social cohesion and, green visibility, and young people receive further benefits from physical activity (PA). These findings offer a comprehensive understanding of the associations and mechanisms between neighbourhood GS and the mental health of disadvantaged groups, addressing health equities that are induced by the unfair distribution of GS, and thus promoting health-oriented environmental planning and policies.

Urban public green spaces are widely recognized for having positive effects on mental health, yet existing research shows imbalances in subjects and methodologies. Most studies examine healthy adults and self-reported indicators, giving limited attention to vulnerable populations; this may have led to a gap in evidence regarding “who is represented and who is overlooked.” This study systematically reviewed 235 empirical papers published in 2004–2024 following PRISMA 2020 to examine epistemic equity. The Equity Bias Framework was applied to operationalize epistemic equity by assessing imbalances in study design, psychometric measures, and population sampling. Results showed that the cross-sectional design, self-report, and community-dwelling adults combination accounted for the largest share (n = 99, 27.8%), whereas only 9 combinations in total (2.5%) involved clinical populations. The experimental design × self-report × university student patterns (n = 14, 3.9%) outnumber all experimental studies involving age-disadvantaged or clinical groups (n = 4, 1.1%). These patterns indicate that existing research evidence is shaped more by feasibility and accessibility than by representativeness and clinical relevance, raising concerns about epistemic equity. By introducing the Equity Bias Framework, this study provides a lens on current evidence and direction for research and policy promoting methodological diversity and sample inclusivity.