This article examines the compelling enigma of how the introduction of a new international law, the North American Agreement on Labor Cooperation (NAALC), helped stimulate labor cooperation and collaboration in the 1990s. It offers a theory of legal transnationalism—defined as processes by which international laws and legal mechanisms facilitate social movement building at the transnational level—that explains how nascent international legal institutions and mechanisms can help develop collective interests, build social movements, and, ultimately, stimulate cross-border collaboration and cooperation. It identifies three primary dimensions of legal transnationalism that explain how international laws stimulate and constrain movement building through: (1) formation of collective identity and interests (constitutive effects), (2) facilitation of collective action (mobilization effects), and (3) adjudication and enforcement (redress effects).