Bainite in steels is conventionally formed by austenite decomposition above the martensite start temperature, Ms. An alternative treatment that has been gaining attention is to quench the steel below Ms, partially transforming the austenite to martensite, and then form bainite by isothermal holding at either the quenching temperature or a temperature above Ms. It is generally accepted that bainite formation in conventional austempering starts by nucleation at the austenite grain boundaries. When the austenite partially decomposes to martensite before bainite formation, new defects—such as dislocations and martensite/austenite interfaces—at which bainite may nucleate are created. Experiments reported in the literature found that in most cases the bainitic reaction can be significantly enhanced by the formation of prior martensite, suggesting that bainite nucleates at these defects. In the present work, a kinetic model of bainite formation, based on the theory of diffusionless bainite growth, has been modified to take into account the nucleation of bainite induced by the presence of prior martensite. Previous models consider only nucleation of bainite at austenite grain boundaries and at bainite/austenite interfaces (autocatalysis). In order to add the effect of prior martensite to the model, we considered that potential sites for bainite nucleation develop at the martensite/austenite interfaces. The simultaneous nucleation at the different types of sites is handled by geometrical assumptions based on the prior austenite grain size, the size of the bainitic subunits, and the area of the martensite/austenite interfaces. A theoretical analysis of the new model showed that it is capable of representing the effect of prior martensite on the kinetics of bainite formation. Finally, the model was validated by comparison against experimental data of bainite formation kinetics in the presence of prior martensite for both low and high carbon steels.