We have discovered periodic light variations (P = 0.1718023 d) in the sdB star HS 2333+3927 in the BVR bands with amplitudes of 0.21. 0.28 and 0.33 mag, respectively. Sinusoidal radial velocity variations at the same period were detected with a semi-amplitude of K-1 = 89.6 km s(-1), indicating that it is binary system and that the light variations are caused by the reflection effect with no eclipses. A mass function of f(m) = 0.0128 M. has been determined. The analysis of the light curve did not yield a unique solution, mainly because the albedo of the secondary is poorly constrained. Two solutions of equal quality with a high (A(2) = 1.0) and a low (A(2) = 0.39) albedo were considered further. Variability of the Balmer line profiles, most notably for Halpha, was discovered. probably also caused by the reflection effect. A spectroscopic analysis results in T-eff = 36 500 K, log g = 5.70, and log (n(He)/n(H)) = -2.15. These characteristics are typical for sdB stars. Mass-radius relations are derived from the results of the analysis of light and radial-velocity curves. Comparison with the observed mass-radius relation of the sdB star and with that of lower main sequence stars for the companion allows us to discard the high albedo solution, because the resulting mass of the primary and the radius of the secondary would be unreasonably low. From a discussion of evolutionary models we constrain the plausible mass of the sdB to the range between 0.29 M. and 0.47 M.. Accordingly, the mass of the secondary is between 0.24 M. and 0.32 M., indicating a spectral type of M3 to M4. HS 2333+3927 is only the sixth sdB+dM system discovered so far. An improved measurement of the gravity and the projected rotational velocity of the sdB star is required to further constrain the masses and to identify the evolutionary state of the sdB star uniquely.