Successful subway construction projects involve many socio-economic, cultural and environmental challenges. These projects enhance public convenience, and are complex and uncertain, involving multiple governmental organizations. The critical success factors (CSFs) of these projects have been identified and prioritized from a main contractors’ perspective, to enable project managers to prioritize their efforts and concerns. This study sheds light on CSFs in subway construction projects and contributes to the project delivery success literature with an overarching focus on the contractors' perspectives. Our investigation revealed that, for subway construction projects in Iran, goal setting through clear and realistic project goals, project management competency, competent project team, good cultural fit, top management support and adequate funding throughout the project are the top-ranking CSFs. An extensive body of literature is reviewed, and 140 potential success factors are identified. Then, a focus group has been conducted in which the potential CSFs reduced to 39 success factors. The priorities and ranking of the success factors were evaluated according to the views of 63 project managers of subway projects. The correlation coefficients analysis was used to analyze the results, and it revealed that 23 success factors such as goal setting, top management support and PM competency were perceived to be critical by the respondents. No other research has, until now, found the critical factors leading to success in subway construction industry in Iran. The results have been verified against other research done in other countries.