We calculate the photon pair production rate in a strong magnetic field created in off-central heavy-ion collisions. Photon decay leads to depletion of the photon yield by a few percent at RHIC and by as much as 20% at the LHC. It also generates a substantial azimuthal asymmetry (“elliptic flow”) of the final photon distribution. We estimate v2 ≈ 2% at RHIC and v2 ≈ 14% at LHC. Photon decay measurements is an important tool for studying the magnetic fields in early stages of heavy-ion collisions.