We report the discovery of an eclipsing binary - HS 0705+6700 - being an sdB star with a faint companion. From its light curve the orbital period of 8263.87 s, the mass ratio of the system q = 0.28, the inclination of 84.degrees4 and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi-amplitude of the radial velocity curve K-1 = 85.8 km s(-1) and a mass function of f(m) =0.00626 M-circle dot are determined. A spectroscopic analysis of the blue spectra results in T-eff = 28 800 K, log g = 5.40, and log(n(He)/n(H)) = 2.68. These characteristics are typical for sdB stars, as is its mass of 0.48 M-circle dot. According to its mass (0.13 M-circle dot) and radius (0.19 R-circle dot), the companion is an M dwarf. The primary is in a core helium burning phase of evolution, and the system must have gone through a common envelope stage when the primary was near the tip of the red giant branch.