A semiconductor device includes a bit line structure on a substrate, a spacer structure including a first spacer directly contacting a sidewall of the bit line structure, a second spacer directly contacting a portion of an outer sidewall of the first spacer, the second spacer including air, and a third spacer directly contacting an upper portion of the first spacer and covering an outer sidewall and an upper surface of the second spacer, and a contact plug structure extending in a vertical direction substantially perpendicular to an upper surface of the substrate and directly contacting an outer sidewall of the third spacer at least at a height between respective heights of a bottom and a top surface of the second spacer.