The influence of spin-orbit interaction in an unoccupied surface state at Ta(110) is investigated with spin- and angle-resolved photoemission and electronic structure calculations. The surface state appears in a symmetry gap at a binding energy of 0.45 eV at [(Γ)] and exhibits a free-electron E(k_||) dispersion with an effective mass m^*/m_e of about -1.35 along [(Γ)] [(H)]. Photoemission results for excitation with s- and p-polarized light confirm the predicted d_z²-type symmetry of the state close to [(Γ)]. Spin-resolved data for finite k_|| reveal a pure Rashaba-type spin texture with a Rashba parameter of 0.063 ± 0.007 eV Å. These findings clearly prove a sizable impact of spin-orbit coupling on the d_z² surface state and resolve a longstanding disagreement on this issue.