The behavior of holes in the valence band of BaTiO3 is investigated using hybrid density-functional calculations. We find that holes tend to self-trap, localizing on individual O atoms and causing local lattice distortions, forming small hole-polarons. This takes place even in the absence of intrinsic defects or impurities. The self-trapped hole (STH) is more energetically favorable than the delocalized hole in the valence band. The calculated emission peak energy corresponding to the recombination of a conduction band electron with a STH can explain the observed photoluminescence at low temperatures. The stability of the STH, its migration barrier, and the related emission peak are then compared to those of SrTiO3. [ABSTRACT FROM AUTHOR]