We have measured the frequency of the 6s(2)S(1/2)(2)-5d D(3/2)(2) electric-quadrupole transition of (171)(Yb) (+) with a relative uncertainty of 1x10(-14) , nu(Yb)=688358 979309312Hz +/-6Hz . We used a femtosecond frequency comb generator to phase-coherently link the optical frequency derived from a single trapped ion to a cesium-fountain-controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on a (171)Yb(+)-ion standard.