Herein, the effect of tempering at 200 °C for 20 min on the mechanical properties of a cold-rolled and intercritical annealed medium Mn steel is investigated in this study. This effect is mainly dependent on the original annealing condition. For the sample annealed at a relatively low temperature (660 °C), the austenite stability of this sample is further improved significantly after tempering. Thus, the transformation-induced plasticity (TRIP) effect is severely inhibited during deformation, which leads to a decrease in the properties of the sample. By contrast, for the sample possessing suitable austenite fraction (%30%) and heterostructure, that is, the sample annealed at 680 °C for 10 min, the simultaneously increased strength and ductility are significantly intrigued by tempering treatment. Electron probe microanalysis and 3D atom probe results reveal that C concentration in austenite is increased due to the C repartition during tempering, which leads to the persistent TRIP effect in a broad strain range during tensile deformation. In addition, the heterogeneous-deformation-induced stress of the heterostructured sample is found to be obviously intensified by tempering treatment. Thus, an excellent strength plastic synergistic effect with a product of ultimate tensile strength and total elongation of approximately 70 GPa% is obtained in the tempered sample.