A semiconductor device includes a gate structure and a contact plug. The gate structure extends in a first direction parallel to the substrate, and includes a first conductive pattern, a second conductive pattern and a gate mask sequentially stacked. The contact plug contacts an end portion in the first direction of the gate structure, and includes a first extension portion extending in a vertical direction and contacting sidewalls of the gate mask and the second conductive pattern, a second extension portion under and contacting the first extension portion and a sidewall of the first conductive pattern, and a protrusion portion under and contacting the second extension portion. A bottom of the protrusion portion does not contact the first conductive pattern. A first slope of a sidewall of the first extension portion is greater than a second slope of a sidewall of the second extension portion.