This meta-analysis examines the relationship between budget participation and budgetary slack by synthesising findings from multiple empirical studies conducted across public and private sector organisations. Drawing on studies with diverse contexts and methodologies, the analysis aims to provide a comprehensive estimate of the overall effect size and to assess potential sector-specific differences. The results reveal a small but statistically significant positive relationship between budget participation and budgetary slack, suggesting that greater involvement in the budgeting process may create opportunities for individuals to introduce slack. Subgroup analysis suggests that this relationship is slightly stronger in the public sector compared to the private sector, although no significant difference was found between the two sectors. Funnel plot analyses show no substantial evidence of publication bias, and the heterogeneity across studies is low, supporting the consistency of the findings. The study contributes to the theoretical understanding by highlighting the complex dynamics of participatory budgeting, where intended benefits, such as enhanced motivation and information sharing, may coexist with unintended behaviours, like slack creation. Practically, the results underscore the importance of coupling participative budgeting processes with appropriate control mechanisms to mitigate the risk of opportunistic behaviour. Limitations of the analysis include measurement inconsistencies, the predominance of cross-sectional data, and the need for deeper exploration of moderating variables. Future research is encouraged to adopt longitudinal designs and to examine contextual and psychological factors that may influence the budget participation–budgetary slack relationship.