ABSTRACT. We present the photometric properties of the eclipsing binary TU Boo from detailed studies of the period and light variations, based on our new BVR observations and historical data collected from the literature. A total of 349 times of minimum light over 80 yr, including our four timings, were used for the period study. An analysis of the resulting diagram reveals that the orbital period of TU Boo has varied in a quasi-sinusoidal O-C way superposed on a long-term period decrease. The secular variation presumably indicates that mass is thermally transferred from the more massive primary star to its companion at a rate of about 7.24 # 10 Ϫ8 M , yr Ϫ1 . The period and semiamplitude of the cyclical variation are 54.5 yr and 0.0177 days, respectively, which could be produced by either a light-time effect or a magnetic activity cycle. Our light curves are asymmetrical, with max I about 0.02-0.03 mag brighter than max II. Among possible spot models, the light variations were bestfit by using a two-spot model with both a cool spot and a hot spot on the secondary star. These may be formed by a combination of effects from a magnetic dynamo and mass transfer. We suggest that TU Boo is probably a triple system, consisting of a massive primary star with a spectral type of G3, a secondary component of spectral type G5-6, and a low-mass M-type tertiary companion with a minimum mass of about p 0.34 M , . M 3