A period study of the binary system TX Her based on the extensive series of minima times covering almost a century (95 years) indicates the periodic O-C variation for the system. This continuous oscillatory variation covers two maxima, about 16800 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 51.47 +/- 0.27 years in the system. The analysis yields a light-time semi-amplitude of 0.0143 +/- 0.0003 days and an orbital eccentricity of 0.81 +/- 0.02. Adopting the total mass of TX Her, the mass of the third body assumed in a co-planar orbit with the binary is M-3 = 0.48 +/- 0.02M. and the semi-major axis of its orbit is a(3) = 18.32 +/- 0.21 AU. (C) 2003 Elsevier B.V. All rights reserved.