Co-based bulk glassy alloys with diameters up to 4mm were formed in a [(Co1−xFex)0.75B0.2Si0.05]96Nb4 system. The bulk glassy alloys exhibit a superhigh fracture strength of 3980–4170MPa and Young’s modulus of 190–210GPa. The bulk glassy alloys also exhibit excellent soft-magnetic properties, i.e., high saturation magnetization of 0.71–0.97T, low coercive force of 0.7–1.8A∕m, high permeability of 1.48–3.25×104, and extremely low saturation magnetostriction of 0.55–5.76×10−6. The first successful synthesis of the Co–Fe–B–Si-based bulk glassy alloys exhibiting superhigh fracture strength and excellent soft-magnetic properties with extremely low magnetostriction is encouraging for future development of Co-based bulk glassy alloys as new engineering and functional materials.