750 GeV diphotons from closed string states

We show that low-mass-scale string compactifications, with a generic D-brane configuration that realizes the standard model by open strings, can explain the relatively broad peak in the diphoton invariant mass spectrum at 750 GeV recently reported by the ATLAS and CMS collaborations. Under reasonable assumptions, we demonstrate that the excess could originate from a closed string (possibly axionic) excitation \varphi that has a coupling with gauge kinetic terms. We estimate the \varphi production rate from photon-photon fusion in elastic pp scattering, using the effective photon and narrow width approximations. For string scales above todays lower limit M_s \approx 7 TeV, we can accommodate the diphoton rate observed at Run II while maintaining consistency with Run I data.