Abstract: Poincare is well known for his conventionalism and structuralism. However, the relationship between these two theses and their place in Poincare’s epistemology of science remain puzzling. In this paper I show the scope of Poincare’s conventionalism and its position in Poincare’s hierarchical approach to scientific theories. I argue that for Poincare scientific knowledge is relational and made possible by synthetic a priori, empirical and conventional elements, which, however, are not chosen arbitrarily. By examining his geometric conventionalism, his hierarchical account of science and defence of continuity in theory change, I argue that Poincare defends a complex structuralist position based on synthetic a priori and conventional elements, the mind-dependence of which precludes epistemic access to mind-independent structures.