Uniqueness results for asymptotically locally flat and asymptotically flat S1-symmetric gravitational instantons are proved using a divergence identity of the type used in uniqueness proofs for static black holes, combined with results derived from the G-signature theorem. Our results include a proof of the S1-symmetric version of the Euclidean Black Hole Uniqueness conjecture, a uniqueness result for the Taub-bolt family of instantons, as well as a proof that an ALF S1-symmetric instanton with the topology of the Chen-Teo family of instantons is Hermitian.