We present time-resolved spectroscopy and photometry of SDSS J100658.40+233724.4, which we have discovered to be an eclipsing cataclysmic variable with an orbital period of 0.18591324 days (267.71507 min). The observed velocity amplitude of the secondary star is 276 ± 7 km s-1, which an irradiation correction reduces to 258 ± 12 km s-1. Doppler tomography of emission lines from the infrared calcium triplet supports this measurement. We have modelled the light curve using the lcurve code and Markov Chain Monte Carlo simulations, finding a mass ratio of 0.51 ± 0.08. From the velocity amplitude and the light curve analysis we find the mass of the white dwarf to be 0.78 ± 0.12 {{M}_&sun;} and the masses and radii of the secondary star to be 0.40 ± 0.10 {{M}_&sun;} and 0.466 ± 0.036 {{R}_&sun;}, respectively. The secondary component is less dense than a normal main sequence star but its properties are in good agreement with the expected values for a CV of this orbital period. By modelling the spectral energy distribution of the system we find a distance of 676 ± 40 pc and estimate a white dwarf effective temperature of 16 500 ± 2000 K. Spectra are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/929