We present time-resolved J-band spectroscopy of the short-period cataclysmic variable SDSS J143317.78+101123.3. We detect absorption lines from the sub-stellar donor star in this system, which contributes 38 ± 5 per cent to the J-band light. From the relative strengths of the absorption lines in the J band, we estimate the spectral type of the donor star to be L2 ± 1. These data are the first spectroscopic detection of a donor with a confirmed sub-stellar mass in a cataclysmic variable, and the spectral type is consistent with that expected from semi-empirical evolutionary models. Using skew mapping, we have been able to derive an estimate for the radial velocity of the donor of K d = 520 ± 60 km s−1. This value is consistent with, though much less precise than, predictions from mass determinations found via photometric fitting of the eclipse light curves.