The notion of noncommutative principal bundle is not as well understood as that of noncommutative vector bundle. Main examples, like the noncommutative instanton bundle, are algebraically understood as Hopf-Galois extensions. This definition is particularly useful when the noncommutative base space is affine (the function algebra being given in terms of generators and relations). We relax it by presenting a sheaf theoretic approach that allows to consider noncommutative principal bundles over non affine base spaces; examples include projective spaces.