This article aims to analyze trade union freedom as a fundamental and human right, based on a comparative approach between the Brazilian and Spanish models, using the International Labour Organization Convention87 as a reference. The study adopts a deductive method, with bibliographic and documentary research. Initially, it addresses the concept of trade union freedom and its philosophical and legal foundations, highlighting its recognition in international treaties such as the Universal Declaration of Human Rights and the UN Covenants, which reinforce its universal and supranational character. Subsequently, the Brazilian union model is examined. Although the 1988 Federal Constitution formally recognizes trade union freedom, it maintains practices such as union unicity and compulsory classification, which are incompatible with International Labour Organization standards. These elements undermine both collective and individual freedoms by limiting union formation and freedom of association. In contrast, the Spanish model, after the end of the Franco regime, underwent reforms that fully guaranteed trade union freedom. The 1978 Constitution, together with Organic Law 11 in 1985, ensures autonomy, pluralism, and free union affiliation, in line with Convention 87. It is concluded that Brazil still faces structural obstacles that prevent its accession to the convention, while the Spanish experience shows that it is feasible to adopt a free and democratic union model, essential to strengthening labor relations and citizenship.