Optical spectroscopy of the blue star KIC 10449976 shows that it is an extremely helium-rich subdwarf with effective temperature Teff = 40 000 ± 300 K and surface gravity log g = 5.3 ± 0.1. Radial-velocity measurements over a 5 d time-scale show an upper variability limit of ≈50 ± 20 km s-1. Kepler photometry of KIC 10449976 in both long and short cadence modes shows evidence for a periodic modulation on a time-scale of ≈3.9 d. We have examined the possibility that this modulation is not astrophysical but conclude that it is most likely real. We discuss whether the modulation could be caused by a low-mass companion, by stellar pulsations or by spots. The identification of any one of these as cause has important consequences for understanding the origin of helium-rich subdwarfs.