Cryo-cooled copper-mold suction casting reveals an unexpected phase separation (PS) in Cu46Zr46Al8 bulk metallic glass (BMG) by controlled atomic diffusion. The decomposed alloy cryo-cast to 193 K exhibits a remarkably lower free volume than the alloy cast to room temperature manifested by dilatometry. An unusual endotherm after the onset of relaxation registered by differential scanning calorimetry and compositional homogenization occurring on long time scales upon post-heat treatment even above the glass-transition temperature confirm the stability of the decomposed state against crystallization. As revealed by scanning transmission electron microscopy and energy-dispersive X-ray analysis, single-stage PS (cryo-cast to 193 K) results in distinct and evenly dispersed nanoscale Cu- and Zr-rich regions, whereas two-stage PS (cast to 298 K) involves further decomposition of existing Cu- and Zr-rich regions on a finer scale. Notably, PS accounts for macroscopic compressive plasticity at no expense of fracture strength. This study opens opportunities for the design optimization of homogeneously dispersed amorphous nanostructures of controllable size and composition in BMGs by single- or multistage decomposition through mold temperature control.