I argue that all rights exist by convention. According to my definition, a right exists by convention just in case its justification appeals to the rules of a socially shared pattern of acting. I show that our usual justifications for rights are circular, that a right fulfills my criterion if all possible justifications for it are circular, and that all existing philosophical justifications for rights are circular or fail. We find three non-circular alternatives in the literature, viz. justifications of rights (...) by consequences, by autonomy or by divine commands. I show that all three alternatives fail, and I conclude that all rights exist by convention. This ontological result has a surprising and beneficial consequence. A common argument against conventionalism is that it implies cultural relativism. I finish by showing that the suggested conventionalism is incompatible with cultural relativism. (shrink)