As demonstrated conceptually by first-principles calculations, the Rashba spin splitting in the 6p states of a Bi adlayer on BaTiO₃(001) can be manipulated by the electric polarization in the ferroelectric substrate. Although this spin-electric effect is moderate, with a relative change in the splitting of about 5%, the absolute splitting of about 0.24 Å-1 is unmatched. Further, the occupied 6p surface states display an anisotropic dispersion and deviate significantly from the free-electron model of the Rashba effect. Our findings may pave a route for spin-electronic devices.