The viscoelastic properties of the zona pellucida (ZP), which is the extracellular coat surrounding an oocyte/embryo, are evaluated in this study. Previous studies demonstrate that ZP mechanical properties change during oocyte maturation, fertilization, and early embryo development, but linear pure elastic models currently being used do not satisfy the time-dependent mechanical behavior of the ZP. In this paper, nonlinear viscoelastic characterization was performed using the Hunt-Crossley model and the newly developed vision-based nanoforce estimation method. The results show that viscoelasticity is a physical property of the ZP that exhibits hysteresis. The stiffness and viscosity parameters simultaneously increase following fertilization, causing the stiffness and viscosity of the embryo ZP (ten samples) to be 2.57-fold and 4.44-fold greater, respectively, than that of the oocyte ZP (eleven samples). This behavior well describes the noncovalently cross-linked filamentous structure of the ZP, supporting zona hardening during fertilization as a mechanically relevant event.