Even though social responsibility (SR) has been widely discussed as a business concept, it is still not being effectively implemented within mega-construction projects due to a range of barriers. Overcoming these barriers requires detailed information, however, there is a lack of research on the barriers to SR implementation within mega-construction projects, particularly in developing countries like the Kingdom of Saudi Arabia (KSA). This study thus aims to investigate the critical barriers to SR implementation with reference to mega-construction projects in the KSA. Eleven barriers were identified from a comprehensive literature review and interviews with experts. These barriers were examined in more detail using a questionnaire survey on two current mega-construction projects in the KSA. There were no differences in the ranking of barriers between the two projects with the 136 respondents identifying the top seven SR barriers as being: (i) additional costs; (ii) lack of awareness and knowledge; (iii) lack of guidelines and coherent strategy; (iv) lack of stakeholder communication; (v) lack of law enforcement; (vi) lack of training; and (vii) unclear project requirements. The findings of this study not only contribute to deeper understanding of the critical SR implementation barriers, but could also encourage industry practitioners and stakeholders to improve SR activities for more effective SR implementation. Moreover, identification and ranking of the critical barriers allows decision-makers to prioritize and develop effective strategies to tackle them at both project and sector scale. Future studies should investigate the interrelationship between the critical barriers and their impact on SR implementation.