A method for manufacturing a group III nitride semiconductor crystal substrate includes providing, as a seed crystal substrate, a group III nitride single crystal grown by a liquid phase growth method, and homoepitaxially growing a group III nitride single crystal by a vapor phase growth method on a principal surface of the seed crystal substrate. The principal surface of the seed crystal substrate is a +c-plane, and the seed crystal substrate has an atomic oxygen concentration of not more than 1×1017 cm−3 in a crystal near the principal surface over an entire in-plane region thereof.