In the historiography of structuralism, Jean Piaget is considered primarily for his work from the 1950s onward, and especially for the publication of his "Que sais-je?" Le structuralisme from 1968. Through source research and analysis of his texts from 1918 to 1968, we show that Piaget adopted a first structural stance as early as the 1920s, which then developed into a "genetic structuralism" in the period 1936-1959. This research program synthesized the results obtained in two areas of study: the first was devoted to logic and focuses on the operative structures of thought, while the second highlighted experimentally the genesis of concepts in children. Piaget's program of "genetic structuralism," though he did not use the term at the time, unfolded his constructivist approach, and his 1968 book, Le structuralisme was paradoxically the place where constructivism was confronted with structuralism. Thus, although Piaget's work from 1936-1959 anticipated French structuralism, its Trojan horse was constructivism, which dialecticized the structural and genetic approaches. This factor, among others, helps to explain the fictitious - because insignificant - place that the French historiography of structuralism has given him.