Short Gamma Ray Bursts are hypothesized to be associated with neutron star mergers. During a merger, the nuclear matter in the two stars achieves super-nuclear densities and moderate temperatures T=20-100 MeV. These density and temperature ranges are expected to be at or near the phase transition from hadron to quark degrees of freedom. For a signal of the transition, we study photon production due to the QCD trace anomaly. Comparing to other sources of photons expected during the merger, we identify a distinguishable feature of the photon spectrum. This feature could serve as a signature of a QCD phase transition in the electromagnetic radiation from neutron star mergers and hence also a possible observational feature in short gamma ray bursts.