This article comments on Castoriadis’s theory of political subject and relates it to its first political context of enunciation, that is, the rise of “new social movements” from the 1960s. The author’s thesis is that Castoriadis’s conception of the political subject as “instituting people” sheds new light on these movements. On the first hand, his conception shows, beyond their heterogeneity, the common reason they share : institutional creativity. On the other hand, it allows to evaluate the ambiguity of their historical evolution, which was characterized by refocusing their logic around individual rights. The first two parts of the article are dedicated to the “instituting people” as opposed to the modern political figure of the “constituting people.” From a perspective that is not only descriptive but both critical and normative, the author concludes by showing how this original figure of the political subject is related to new social movements.