We report on the discovery of a rapidly rotating disk in a K-selected galaxy at z=1.34. Spatially resolved kinematics are determined from deep, moderate-resolution Keck spectroscopy covering the redshifted [O II] 3727 Å line. A conservative estimate of the maximum rotation velocity Vmax=290±20 km.s–1, placing the galaxy among the most rapid rotators known. A Hubble Space Telescope Wide Field Planetary Camera 2 image taken in the IF814W filter (rest-frame U) shows that the [O II] emission originates in a ring of star-forming regions ∼3" (∼20 h–175 kpc) across. In the K band, the galaxy is compact, and its position coincides with the center of the star-forming ring seen in the rest-frame near-UV. The galaxy has MV=-22.4±0.2 (∼3L*) in the rest frame, and its spectral energy distribution is very well fitted by that of a redshifted Sb/c galaxy. The observed kinematics, morphology, and spectral energy distribution are consistent with a massive bulge-disk system at z=1.34. The lower limit on Vmax gives an upper limit on the offset from the present-day Tully-Fisher relation. The galaxy is overluminous by less than 0.7±0.4 mag in rest-frame V, consistent with previous studies of disk galaxies at lower redshift. Taken together, these results suggest that some of the most luminous spiral galaxies in the nearby universe were already in place ∼1010 yr ago, placing a constraint on models for their formation.