Automobile industries require advanced high-strength steels (AHSSs) that possess high strength and good formability and are light; automobiles manufactured using AHSSs have reduced fuel consumption and enhanced safety compared to automobiles manufactured using traditional materials. Al-containing medium Mn steels, which are a typical example of 3rd generation AHSSs, have attracted much research attention with an aim of meeting these requirements as they possess extraordinary work hardening ability, which leads to high strength and excellent elongation at a low density. However, such steels often exhibit low yield strength resulting from the formation of coarse δ-ferrite grains due to the high Al content. In this study, the influence of tempering temperatures on the microstructure and mechanical properties of hot-rolled medium Mn steel containing 15% (volume fraction) δ-ferrite due to the addition of 3%Al (mass fraction) is studied. δ-ferrite with a length of 300 μm was refined and divided into a large number of bamboo-like grains having a length of about 3 μm due to dynamic recrystallization caused by hot rolling. The grain size of these refined δ-ferrite grains remained unchanged when the tempering temperature was increased to 700^oC. In the case of tempering at 400-500^oC, although the dislocation density in martensite decreased, the precipitation of fine cementite and nanosized VC particles compensated for this effect, leading to high yield strengths, which was almost the highest among all the tempering temperatures. Meanwhile, many C atoms could be partitioned from martensite to austenite, leading to the steel acquiring the enhanced chemical stability of austenite, which contributed to the higher work hardening rate and more durable strain hardening. Finally, the best mechanical combination consisting of a yield strength of about 1500 MPa, an ultimate tensile strength of 1800 MPa, and a total elongation of 14% was achieved after tempering at 400-500^oC. The resultant yield strength is much higher than that of other medium Mn steels having similar Al content because it is dependent on the tempered martensitic matrix rather than δ-ferrite. This is due to two factors: first, δ-ferrite in the studied steel is strengthened due to precipitation, dislocation, and grain refinement hardening; second, δ-ferrite grains have a small fraction of 15% and a refined size of 3 μm; thus, they are actually embedded in the martensite matrix as isolated islands. These results open a path for the designing and manufacturing of new low-density steels having high yield strengths.