Aims.Our aim is to confirm the nature of the long period radial velocity measurements for β Gem first found by Hatzes & Cochran (1993). Methods: .We present precise stellar radial velocity measurements for the K giant star β Gem spanning over 25 years. An examination of the Ca II K emission, spectral line shapes from high resolution data (R = 210 000), and Hipparcos photometry was also made to discern the true nature of the long period radial velocity variations. Results: . The radial velocity data show that the long period, low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. Furthermore, the Ca II K emission, spectral line bisectors, and Hipparcos photometry show no significant variations of these quantities with the radial velocity period. An orbital solution assuming a stellar mass of 1.7 M_&sun; yields a period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis, a = 1.6 AU. The orbit is nearly circular (e = 0.02). Conclusions: .The data presented here confirm the planetary companion hypothesis suggested by Hatzes & Cochran (1993). β Gem is one of six intermediate mass stars known to host a sub-stellar companion and suggests that planet-formation around stars much more massive than the sun may common.