Let G be a semisimple Lie group. We describe the irreducible representations of G by linear isometries on \(L_p\) -spaces for \(p\in (1,+\infty )\) with \(p\ne 2\) . More precisely, we show that, for every such representation \(\pi ,\) there exists a parabolic subgroup Q of G such that \(\pi \) is equivalent to the natural representation of G on \(L_p(G/Q)\) twisted by a unitary character of Q. When G is of real rank one, we give a complete classification of the possible irreducible representations of G on an \(L_p\) -space for \(p\ne 2,\) up to equivalence.