The spine naturally has a nonlinear force-deflection characteristic which facilitates passive stability, and thus there is a need for spinal implants that duplicate this behavior to provide stabilization when the spine loses stiffness through injury, degeneration, or surgery. Additionally, due to the complexity and variability in the mechanics of spinal dysfunction, implants could potentially benefit from incorporating a customizable stiffness into their design. This paper presents a spinal implant with contact-aided inserts that provide a customizable nonlinear stiffness. An analytical model was utilized to optimize the device design, and the model was then verified using a finite element model. Validation was performed on physical prototypes, first in isolation using a tensile tester and then using cadaveric testing on an in-house spine tester. Testing confirmed the performance of the implant and it was observed that the device increased mechanical stability to the spinal segment in flexion-extension and lateral-bending.