We discuss the orbital period changes of the Algol semi-detached eclipsing binary DI Peg by constructing the (O-C) residual diagram via using all the available precise minima times. We conclude that the period variation can be explained by a sine-like variation due to the presence of a third body orbiting the binary, together with a long-term orbital period increase (dP/dt=0.17 sec/century) that can be interpreted to be due to mass transfer from the evolved secondary component (of rate $1.52{ imes}10^{-8}M_{odot}/yr$) to the primary one. The detected low-mass third body ($M_{3min.}=0.22{pm}0.0006M_{odot}$) is responsible for a periodic variation of about 55 years light time effect. We have determined the orbital parameters of the third component which show a considerable eccentricity $e_3=0.77{pm}0.07$ together with a longitude of periastron ${omega}_3=300^{circ}{pm}10^{circ}$.