We report the detection of an extrasolar planet orbiting ∊ Tau, one of the giant stars in the Hyades open cluster. This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594.9+/-5.3 days, a semiamplitude of 95.9+/-1.8 m s-1, and an eccentricity of 0.151+/-0.023. The minimum mass of the companion is 7.6+/-0.2 MJ, and the semimajor axis is 1.93+/-0.03 AU adopting a stellar mass of 2.7+/-0.1 Msolar. The age of 625 Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 Msolar for the host star is robustly determined by isochrone fitting, which makes the star the heaviest among planet-harboring stars. Putting together the fact that no planets have been found around about 100 low-mass dwarfs in the cluster, the frequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones.