BC Cygni (BC Cyg, HIP 100404, BD + 37 3903) is a red supergiant and pulsating variable star of spectral type M3.5Ia in the constellation Cygnus.

It is considered a member of the stellar Cygnus OB1 association, and within it the open cluster Berkeley 87,[9] which would place at a distance of 1,673 parsecs (5,000 ly) of the Solar System;[10] it is less than a degree north of another variable red supergiant, BI Cygni.

[7][a] If it were in the place of the Sun, its photosphere would engulf the entire inner solar system and reach close to the orbit of Jupiter.

[12] Louisa Wells discovered that the star's brightness varied, based on the examination of 15 photographic plates.

[15] The brightness of BC Cyg varies from visual magnitude +9.0 and +10.8 with a period of 720 ± 40 days.