To provide a semiconductor device in which resistance of a source region and a drain region of a thin film transistor is reduced and a short channel effect is suppressed, and a manufacturing method thereof. The semiconductor device includes a gate electrode which is formed over a first semiconductor layer with a gate insulating film interposed therebetween; sidewalls which are formed on side surfaces of the gate electrode; and second semiconductor layers which are in contact with and stacked over end portions of the sidewalls and the first semiconductor layer, wherein the second semiconductor layers cover at least a part of the end portions of the sidewalls.