Retaining walls have a critical role in infrastructural systems and are widely constructed in seismically active regions. To prevent earthquake hazards, the seismic performance of the retaining wall can be improved using cost-effective, sustainable, and easily applicable methods. Using lightweight materials behind the retaining wall is one of these innovative methods. EPS geofoam and tire waste-sand mixture are widely used materials as cushion materials. These lightweight materials have advantageous features such as elastic, compressible, highly permeable, vibration-absorbent, and durable. This study consists of a numerical study using PLAXIS 2D to examine the effects of the density of the cushion on the seismic performance of a retaining wall. EPS Geofoam is used as a compressible layer behind the retaining wall. The effects of cushion density will be investigated under two different earthquake motions. The results are evaluated by comparing the numerical models with three different Geofoam densities under two different earthquake motions. In addition, models with and without the cushion are compared. The numerical studies revealed that the EPS Geofoam density may be a significant factor for improving the seismic performance of the retaining wall.