Ab initio averaged relativistic effective core potentials, spin-orbit operators, and valence basis sets are reported for the fourteen lanthanide elements Ce through Lu in the form of expansions in Gaussian-type functions. Gaussian basis sets with atomic orbital coefficients for low energy states of each atom are also tabulated. Hartree-Fock orbital energies calculated in conjunction with the relativistic effective core potentials using the j-j coupling scheme agree to within 10% of those due to numerical all-electron Dirac-Fock calculations. Excitation energies, ionization potentials, and spin-orbit splittings are compared to all electron, energy-adjusted effective core potential, and experimental results.