We studied electronic structure and Rashba-type spin splitting in the (3×1) and (1×1) structures on Tl/Ge(111) using angle-resolved photoelectron spectroscopy (ARPES) and first-principles calculation incorporating the spin-orbit interaction. The calculations showed that Tl behaves as a monovalent ion and surface states involved with the Tl-Ge bond are formed mainly by the Tl 6p orbital and the Ge dangling bond. The surface states show k-dependent spin splitting up to ∼200 and ∼800 meV for the (3×1) and (1×1) structures, respectively. The occupied surface states were observed by ARPES. Their spin splittings were predicted to be 150−200 meV, while these were not resolved in the measurement. The largest splitting of ∼800 meV is calculated for the unoccupied states at K⁻ on the (1×1) structure. The wavefunction is strongly localized in the Tl overlayer, indicating that the heavy core potential of Tl is essential for the large spin splitting.