Scholars interested in labour in Latin America have traditionally paid little attention to trade unions' legal mobilisation. However, the increasing number of legal complaints filed by workers with labour ministries and/or the courts in countries like Argentina, Brazil and Chile calls for a more serious debate on the role that trade unions play in this process. This article focuses on the Chilean case. Drawing on various sources, it shows that Chilean unions have turned legal complaints into a weapon to gain more rights and curb employers' power. This process has involved the strongest and most combative unions, and is due to two historical conditions: (1) the obstacles placed in the way of successful resort to more disruptive tactics; (2) the increase in institutional opportunities to report infringements of the law. Overall, the article challenges the current image of the Chilean unions by foregrounding their agency and their achievements over the last decade.