BI Cygni (BI Cyg, IRC +40408, BD+36 4025) is a red supergiant in the constellation Cygnus. It is an irregular variable star with a maximum brightness of magnitude 8.4 and a minimum of magnitude 9.9. It is considered a member of the Cygnus OB1 stellar association, its distance is around 1300pc of the Solar System. It is less than a degree south of another variable red supergiant, BC Cygni.
BI Cyg is a slow irregular variable star classified as type Lc, an irregular supergiant. Its brightness changes between extremes of magnitude 8.4 and 9.9. Frequency analysis of its light curve shows no significant periods.
BI Cyg is one of the largest known stars with a radius around, measured by its angular diameter by the CHARA array. It is about 90,000 times more luminous that the Sun and has a cool effective temperature of . Its mass is estimated at 17solar masses, and it took 12 million years to enter the red supergiant phase.