Due to their bioinert nature, titanium alloys show poor bone-implant integration and insufficient osseointegration in vivo. In this study, a series of low elastic modulus bioactive titanium alloys with a nominal composition of Ti-13Nb-13Zr-1.25 Mg (wt%) were prepared using mechanical alloying and spark plasma sintering techniques. The microstructures, mechanical properties, degradation behaviors and in vitro bioactivities of these alloys were systematically investigated. After sintering at 700 °C, the α-Ti, β-Ti and Nb (Zr)-rich phases were present, and the Mg was uniformly distributed. In addition to above-mentioned phases, the α″ phase was found after sintering at 800 °C or 900 °C. The density, elastic modulus, yield strength, ultimate compressive strength and corrosion resistance all increased with increasing sintering temperature. After sintering at 900 °C, the alloy exhibited high density (99.8%), good compressive strength (1417.2 MPa) and excellent corrosion resistance. In addition, it had a lower elastic modulus (~69 GPa) than that of the biomedical alloy Ti–13Nb–13Zr (~80 GPa). In vitro experiments showed that the alloys sintered at either 800 °C or 900 °C promoted cell adhesion and proliferation. However, the alloy sintered at 700 °C inhibited cell proliferation, which was due to the greater release of Mg ²⁺. Thus, the optimally-processed Ti-Nb-Zr-Mg alloy sintered at 900 °C shows immense potential as a biomedical material.