The adult male barn swallow of the nominate subspecies H. r. rustica is 17–19 cm long including 2–7 cm of elongated outer tail feathers. It has a wingspan of 32–34.5 cm and weighs 16–22 g. It has steel blue upperparts and a rufous forehead, chin and throat, which are separated from the off-white underparts by a broad dark blue breast band. The outer tail feathers are elongated, giving the distinctive deeply forked "swallow tail". There is a line of white spots across the outer end of the upper tail. The female is similar in appearance to the male, but the tail streamers are shorter, the blue of the upperparts and breast band is less glossy, and the underparts paler. The juvenile is browner and has a paler rufous face and whiter underparts. It also lacks the long tail streamers of the adult.
The song of the barn swallow is a cheerful warble, often ending with su-seer with the second note higher than the first but falling in pitch. Calls include witt or witt-witt and a loud splee-plink when excited (or trying to chase intruders away from the nest) The alarm calls include a sharp siflitt for predators like cats and a flitt-flitt for birds of prey like the hobby. This species is fairly quiet on the wintering grounds