Provided are a thin film transistor and a method for manufacturing the same. The thin film transistor manufacturing method includes forming a gate electrode on a substrate, forming an active layer that is adjacent to the gate electrode and includes an oxide semiconductor, forming an oxygen providing layer on the active layer, forming a gate dielectric between the gate electrode and the active layer, forming source and drain electrodes coupled to the active layer, forming a planarizing layer covering the gate electrode and the gate dielectric, forming a hole exposing the active layer, and performing a heat treatment process onto the planarizing layer in an atmosphere of oxygen.