We use high dynamic range , high-resolution L -band spectroscopy to measure the radial velocity variations of the hot Jupiter in the \tau Boötis planetary system . The detection of an exoplanet by the shift in the stellar spectrum alone provides a measure of the planet ’ s minimum mass , with the true mass degenerate with the unknown orbital inclination . Treating the \tau Boo system as a high flux ratio double-lined spectroscopic binary permits the direct measurement of the planet ’ s true mass as well as its atmospheric properties . After removing telluric absorption and cross-correlating with a model planetary spectrum dominated by water opacity , we measure a 6- \sigma detection of the planet at K _ { p } = 111 \pm 5 km / s , with a 1- \sigma upper limit on the spectroscopic flux ratio of 10 ^ { -4 } . This radial velocity leads to a planetary orbital inclination of i = 45 ^ { +3 } _ { -4 } degrees and a mass of M _ { P } = 5.90 ^ { +0.35 } _ { -0.20 } M _ { Jup } . We report the first detection of water vapor in the atmosphere of a non-transiting hot Jupiter , \tau Boo b .