Given the growing importance of urban governance and citizen participation in enhancing the quality of urban environments, this study investigates the role of urban governance in improving urban design quality, with particular emphasis on the mediating role of citizen participation in the city of Qom, Iran. The research is applied in terms of purpose and adopts a descriptive-correlational design using Structural Equation Modeling (SEM). The statistical population comprised residents of Qom, from whom 384 respondents were selected through Cochran’s formula using a simple random sampling method. Data were collected through a structured questionnaire and analyzed using SPSS and AMOS software. The validity of the instrument was confirmed through face, content, and construct validity, while its reliability was established using Cronbach’s alpha and composite reliability. The findings revealed that urban governance has a positive and significant effect on citizen participation (standardized coefficient = 0.62) and on urban design quality (standardized coefficient = 0.58). Furthermore, citizen participation positively influences urban design quality (standardized coefficient = 0.47) and serves as a significant partial mediator in the relationship between urban governance and urban design quality, with an indirect effect of 0.29. In addition, urban governance explained 38% of the variance in citizen participation, while urban governance together with citizen participation accounted for 64% of the variance in urban design quality. The results suggest that achieving high-quality urban design in Qom requires the establishment of transparent, accountable, coordinated, and equity-oriented urban governance, accompanied by meaningful citizen participation throughout the processes of urban planning, decision-making, and project implementation.