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Based on three ethnographic studies carried out in the regions of Calais, Briançon, and Menton-Ventimiglia, the article analyzes the difficulties, opportunities, and ambivalences faced by actors involved in solidarity with illegalized foreigners at French borders, while using the law as a repertoire of contention. By doing so, it aims to contribute to two distinct fields of academic thought: one focusing on the relationship between the law and social movements and the other grasping the role of non-state and associative actors in the implementation of immigration control policies. By revitalizing the classical debate on the potential and limits of the law as a weapon for social movements, and as an instrument of social and political change, the article exposes the specificities as well as the ambiguities of activists’ uses of the law in solidarity with migrants at borders.