The effect of nanoanatase titanium dioxide (TiO2) powder on early-age hydration kinetics of tricalcium silicate (C3S) was investigated. Isothermal calorimetry was performed on C3S pastes with 0%, 5%, 10%, and 15% of TiO2 addition by weight, and two mathematical models-the Avrami (or JMAK) model and the boundary nucleation model (BN model)-were fitted to the data. For all of the mixes, the addition of TiO2 increased the peak reaction rate, and increased the degree of hydration at 12 and 24 h. The rate of hydration of 10% and 15% TiO2 pastes were accelerated, while the 5% TiO2 paste was delayed, lengthening the induction period as compared with the control paste. The model fits demonstrate that the BN model captures the kinetics of the reaction better, particularly in the deceleration period, than the Avrami model. This is related to the ratio of rate parameters (k(B)/k(G)) of the BN model, that the increasing ratio gives a more unsymmetrical shape of a rate curve. The increase in k(B)/k(G) with TiO2 addition at 5%, 10%, and 15% suggests that hydration product is formed on or near the surfaces of TiO2 particles, as well as on the C3S surface. These results demonstrate that the addition of TiO2 nanoparticles accelerates the early hydration by providing additional nucleation sites, forming the foundation for future optimization of photocatalytic and other nanoparticle-containing cements.