The paper considers the influence of magnetic phase separation on the magnitude of the magnetocaloric effect. A general thermodynamic generalized Landau’s theory with a variable number of particles is proposed, which allows a simple and consistent description of the first-order phase transition between magnetically ordered and disordered phases, considering phase separation. The calculation of the magnetic susceptibility and entropy of the phases involved in phase separation is considered. It is shown that the magnetic susceptibility of the magnetically ordered (disordered) phase involved in phase separation is negative (positive) in the vicinity of the tricritical point, which can lead to an inversion of the sign of the magnetocaloric effect.