We present a study of the generalized second law of thermodynamics in the scope of the f (R, T ) theory of gravity, with R and T representing the Ricci scalar and trace of the energy-momentum tensor, respectively. From the energy-momentum tensor equation for the f (R, T ) = R + f (T ) case, we calculate the form of the geometric entropy in such a theory. Then, the generalized second law of thermodynamics is quantified and some relations for its obedience in f (R, T ) gravity are presented. Those relations depend on some cosmological quantities, as the Hubble and deceleration parameters, and also on the form of f (T ).