A semiconductor device includes an active pattern on a substrate, a gate structure buried at an upper portion of the active pattern, a bit line structure on the active pattern, a spacer structure on a sidewall of the bit line structure, a contact plug structure contacting the spacer structure, an insulating interlayer structure partially penetrating through upper portions of the contact plug structure, the spacer structure and the bit line structure, and a capacitor on the contact plug structure. The spacer structure includes an air spacer including air. The insulating interlayer structure includes first and second insulating interlayers. The second insulating interlayer may include an insulation material different from that of the first insulating interlayer. A lower surface of the second insulating interlayer covers a top of the air spacer, and a lowermost surface of the first insulating interlayer is covered by the second insulating interlayer.