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Monolayer FeSe on a SrTiO₃ (STO) substrate is a high-temperature superconductor with reported T_c as high as 100 K, but the mechanism for such enhanced T_c remains poorly understood. Here we characterize the atomic structural and chemical composition of the FeSe/STO interface using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Our measurements reveal the presence of selenium in the top layers of STO, located on interstitial sites and in the TiO₂ layers. We support our measurement with density functional theory (DFT) calculations. We discuss implications of our findings on substrate-induced electron doping in the FeSe/STO heterostructure.