A thin film semiconductor device including a thin film transistor (TFT) that maintains a constant electrical characteristic and an organic light-emitting display device. The thin film semiconductor device includes: a substrate; and a thin film transistor (TFT) disposed on the substrate and comprising a semiconductor layer comprising a source region and a drain region, wherein a part of the source region is spaced apart from the drain region and partially surrounds the drain region, and wherein a part of the drain region is spaced apart from the source region and partially surrounds the source region.