In this research a variable-stiffness compliant mechanism was developed to generate variable force-displacement profiles at the mechanisms coupler point. The mechanism is based on a compliant Roberts straight-line mechanism, and the stiffness is varied by changing the effective length of the compliant links with an actuated slider. The variable-stiffness mechanism was used in a one-degree-of-freedom haptic interface to demonstrate the effectiveness of varying the stiffness of a compliant mechanism. Unlike traditional haptic interfaces, in which the force is controlled using motors and rigid links, the haptic interface developed in this work displays haptic stiffness via the variable-stiffness compliant mechanism. The force-deflection behavior of the mechanismwas analyzed using the Pseudo-Rigid Body Model (PRBM), and two key parameters, KQ and g,were optimized using finite element analysis (FEA) to match the model with the behavior of the device. One of the key features of the mechanism is that the inherent return-to-zero behavior of the compliant mechanism was used to provide the stiffness feedback felt by the user. A prototype haptic interface was developed capable of simulating the force-displacement profile of Lachmans Test performed on an injured ACL knee. The compliant haptic interface was capable of displaying stiffnesses between 4200 N/m and 7200 N/m.