The Socio-Labour Declaration is the legal instrument that protects fundamental labour rights within Mercosur and its Member States legal orders. Its 2015 revision enhanced quantitively and qualitatively the rights enshrined therein. Relying upon recent literature on Latin American regional integration, this article considers the complex institutional and legal framework in which the Declaration has been adopted and implemented. It examines how the intergovernmental character of Mercosur has shaped the legal content of the Socio-Labour Declaration. The institutional context of the Declaration requires the active cooperation and intervention of both regional and national actors. This article explores how Mercosur bodies have taken advantage of the flexible institutional framework to implement the Declaration through regional plans and policies. It also analyses the contrasting enforcement roles of the national executive and legislative powers, characterized by their timidity, and the judicial activism that is essential to consider the Declaration as a justiciable instrument. The article concludes that the Socio-Labour Declaration is a crucial instrument in protecting workers’ rights in this trade bloc, and that the 2015 revision introduced substantial improvements that may provide the legal basis for future judgments, and regional and national labour laws reforms.