We present a detectable and unambiguous feature of a strong first-order phase transition (PT) from hadronic to deconfined quark matter encoded in the gravitational wave (GW) signal of a binary neutron star merger. If the PT takes places after the merger it can increase the dominant postmerger frequency relative to the tidal deformability, a quantity, inferred from the premerger signal. We also observe a tight correlation between the dominant postmerger GW frequencies and the largest densities in the system shortly after the merger. By combining both findings we demonstrate that a single, accurate measurement of pre- and postmerger gravitational waves has the potential to yield constraints on the onset density of quark deconfinement.