A period study of the young binary AR Aur based on the extensive series of published photoelectric/ccd minima times indicates the cyclic (O - C) variation for the system. This continuous oscillatory variation covers almost three cycles, about 6000 orbital periods, by the present observational data. It can be attributed to the light-time effect due to a third body with a period of 23.68+/-0.17 years in the system. The analysis yields a light-time semi-amplitude of 0.0084+/-0.0002 day and an orbital eccentricity of 0.20+/-0.04. Adopting the total mass of AR Aur, the mass of the third body assumed in the co-planar orbit with the binary is M-3 = 0.54 +/- 0.03 M-circle dot and the semimajor axis of its orbit is a(3) = 13.0 +/- 0.2 AU.