A new Mg–Cu–Ni–Ag–Zn–Y–Gd alloy with significantly improved glass forming ability (GFA) was developed in this work. Mg65Cu7.5Ni7.5Ag5Zn5Y5Gd5 bulk metallic glass (BMG) with a diameter of 14 mm was successfully fabricated by conventional copper mold casting method in air atmosphere. The critical cooling rate for glass formation was estimated to be about 20 K/s. When the maximum diameter for glass formation was compared with those previously reported for Mg-based alloys, the Mg65Cu7.5Ni7.5Ag5Zn5Y5Gd5 alloy showed the highest GFA. However, when the GFA was compared with those of Mg-TM-RE alloys (TM: Cu, Ni, Zn, and Ag; RE: Y and Gd), the significant improved GFA of the Mg–Cu–Ni–Zn–Ag–Y–Gd alloy cannot be properly represented by ΔTx, Trg, K, and γ parameters. In contrast to most of the Mg-based BMGs reported so far, the Mg65Cu7.5Ni7.5Ag5Zn5Y5Gd5 BMG exhibits yielding and plastic deformation during compressive loading.