Due to the growing depletion of natural resources, conventional materials are gradually being replaced by waste materials. One such waste is steel slag obtained from steel industries. Recent studies have highlighted the promising performance of slag as a fill material in geotechnical structures such as Mechanically Stabilized Earth (MSE) walls and slopes. The present study aims to determine Hyperbolic and Hardening Soil (HS) model parameters of steel slag in unreinforced and geogrid reinforced conditions. Determined model parameters could be easily implemented in various finite element and finite difference softwares to simulate the behavior of slag in aforesaid geotechnical applications. In this regard, a series of strain-controlled static triaxial tests were carried out. Results indicated that reinforcing slag with the geogrid increases the peak strength and axial strain at failure. Results obtained from the experimental studies were approximated using the Hyperbolic model to calculate the model parameters. The Hyperbolic model was able to capture the behaviour of both unreinforced and reinforced slag up to failure. However, the model failed to capture the post-peak behaviour. Therefore, the Hyperbolic model parameters were further utilized to derive the parameters of the HS model. The HS model was found to supersede the Hyperbolic model by overcoming its limitations.