We present high time-resolution spectroscopy of the eclipsing cataclysmic variable SDSSJ143317.78+101123.3 obtained with QUCAM2, a high-speed/low-noise electron-multiplying CCD camera. Littlefair et al. measured the mass of the secondary star in SDSSJ143317.78+101123.3 using a light-curve fitting technique and obtained a value of M2 = 0.060 +/- 0.003Msolar, making it one of the three first bona fide detections of a brown dwarf mass donor in a cataclysmic variable. In this Letter, we present a dynamical measurement supporting this important result. We measured the radial velocity semi-amplitude of the white dwarf from the motion of the wings of the Hα emission line and obtained a figure of K1 = 34 +/- 4kms-1, in excellent agreement with the value of K1 = 35 +/- 2kms-1 predicted by Littlefair et al.'s model.