AbstractPublic–Private Partnership (PPP) programs have been developed widely for more than 30 years across the world. The continuity of these programs depends on an adequate balance of three-dimensional sustainability (i.e., economic, environmental, and social). Nevertheless, social sustainability has been demonstrated as being fragile because of the challenges to achieving the intended social legitimacy in PPP programs. This study aims to understand key challenges in achieving social legitimacy in road PPP programs by analyzing contractual clauses, legal frameworks, and stakeholders’ interviews of two toll road PPP programs in Chile and Colombia. Three key challenges to social legitimacy were found: social involvement issues, distrust between impacted and responsible stakeholders, and lack of social criteria within toll tariff policy. Findings reveal that it is required to move beyond current consultation mechanisms and thoroughly involve the impacted groups as a relevant stakeholder typology in order to maximize value creation in user-pay PPP programs. In line with that, this study exposes that the claim of previous research for developing relational governance between the public and the private sectors is not enough for overcoming governance limitations and addressing social legitimacy in user-pay PPPs. It is also necessary to enhance relational governance in a triadic approach. This study contributes to the PPP body of knowledge by redirecting the discussion from overall legitimacy to social legitimacy challenges and by including the impacted stakeholders in the analysis of PPP governance mechanisms.