We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 (V406 Vul). Photometric observations taken during 2000 and 2008 reveal the presence of the secondary star's ellipsoidal modulation. OSIRIS spectra obtained with the GTC in 2010 reveal radial velocity variations of $\sim 500$ km s$^{-1}$ over 3 h. A simultaneous fit to the photometry and spectroscopy using sinusoids to represent the secondary star's ellipsoidal and radial velocity variations, yields an orbital period of $6.58 \pm 0.05$ h and a secondary star's radial velocity semi-amplitude of $K_2= 541 \pm 70$ km s$^{-1}$. The implied mass function is $f(M) = 4.5 \pm 0.6$ M$_\odot$, significantly lower than previously reported but consistent with the presence of a black hole in XTE J1859+226. The lack of eclipses sets an upper limit to the inclination of 70 degrees which yields a lower limit to the black hole mass of 5.42 M$_\odot$.