Abstract: Although the existence of glass–glass interfaces (GGIs) enables improved ductility of metallic nanoglasses (NGs), the excess free volumes at GGIs would cause the NGs to have a much-reduced mechanical strength. Herein, entropy-stabilized GGIs have been investigated in Co–Fe–Ni–Zn–P NGs, which have a large entropy of mixing (1.32R, where R is the gas constant) and could be in a new glass phase, different from that of glassy grain interiors. Through quantitatively determining the activation energy of glass transition separately for the GGIs and glassy grain interiors, the excess free volumes at GGIs are found to be reduced in comparison with those in the glassy grain interiors. The thermodynamically stable GGIs could be associated with increasing entropy of mixing in the GGI regions, which stabilizes the atomic structures of GGIs and enhances the glass forming ability of Co–Fe–Ni–Zn–P NGs. The influences of entropy-stabilized GGIs on the mechanical properties of Co–Fe–Ni–Zn–P NGs are further investigated by nanoindentation and creep tests under tensile deformation, demonstrating that there are notable enhancements in the ductility and mechanical strength for Co–Fe–Ni–Zn–P NGs. This work contributes to an in-depth understanding on the GGI phase in NGs and offers an alternative method for strengthening NGs through GGI engineering.