The described technology relates generally to a thin film transistor for a display device and an organic light emitting diode display device including the same. An exemplary embodiment provides a thin film transistor for a display device, including: a substrate; a semiconductor that is disposed on the substrate and includes a channel, and a source region and a drain region disposed at opposite sides of the channel; a gate insulating layer that includes a first gate insulating layer disposed on the substrate and the semiconductor, and a second gate insulating layer disposed on the first gate insulating layer and overlapping the channel; a gate electrode disposed on the second gate insulating layer; an interlayer insulating layer disposed directly on the first gate insulating layer and the gate electrode; and a source electrode and a drain electrode that are disposed on the interlayer insulating layer and are connected to the semiconductor, wherein a thickness of a portion of the gate insulating layer overlapped with the gate electrode may be larger than that of a portion of the gate insulating layer overlapped with the source region and that of a portion of the gate insulating layer overlapped with the drain region.