  In this letter we begin the study of visible dark sector signals coming from
binary neutron star mergers. We focus on dark photons emitted in the 10 ms - 1
s after the merger, and show how they can lead to bright transient gamma-ray
signals. The signal will be approximately isotropic, and for much of the
interesting parameter space will be close to thermal, with an apparent
temperature of about $100$ keV. These features can be used to distinguish the
dark photon signal from the expected short gamma-ray bursts produced in neutron
star mergers, which are beamed in a small angle and non-thermal. We calculate
the expected signal strength and show that for dark photon masses in the
$1-100$ MeV range it can easily lead to total luminosities larger than
$10^{46}$ ergs for much of the unconstrained parameter space. This signal can
be used to probe a large fraction of the unconstrained parameter space
motivated by freeze-in dark matter scenarios with interactions mediated by a
dark photon in that mass range. We also comment on future improvements when
proposed telescopes and mid-band gravitational detectors become operational.
