The present invention relates to a thin film transistor, a thin film transistor array panel, and a manufacturing method thereof. A thin film transistor according to an exemplary embodiments of the present invention includes: a gate electrode; a gate insulating layer positioned on or under the gate electrode; a channel region overlapping the gate electrode, the gate insulating layer interposed between the channel region and the gate electrode; and a source region and a drain region, facing each other with respect to the channel region, positioned in the same layer as the channel region, and connected to the channel region, wherein the channel region, the source region, and the drain region comprise an oxide semiconductor, and wherein a carrier concentration of the source region and the drain region is larger than a carrier concentration of the channel region.