We demonstrate by first-principles calculations that the Rashba spin-orbit splitting in the 6p states of a Bi adlayer on BaTiO₃(001) is strongly affected by the substrate termination. For the TiO₂ termination the absolute splitting is very large (about 0.23 Å⁻¹); in contrast, the splitting becomes strongly reduced for the BaO termination (less than 0.07 Å⁻¹). Our findings are explained by the termination-dependent hybridization of the Bi surface states with electronic states in the substrate.