This paper presents the results of an experimental investigation of the dynamic properties of two expanded polystyrene (EPS) geofoam materials. Torsional resonant column tests and cyclic uniaxial tests were conducted on block-molded EPS geofoam specimens with mean densities of 12.4 and 17.1 kg/m 3 , under zero confining pressure. Theestimated dynamic moduli values (shearmodulusandmodulusofelastic- ity) and damping ratio values are presented as functions of cyclic strain amplitude and loading frequency. The geofoam behavior for strains ranging from 5 × 10 -4 to 8.0% was studied by combining the results of the two types of tests. It was found that EPS geofoambehaveslinearly forstrainvalues ofupto0.1%; however,astrongnonlinearity develops forstrains greater than 1%.Thegeofoam density significantly affected thedy- namic moduli values (the moduli increase with increasing density) whereas no appre- ciable effect on damping ratio values was found. The damping ratio values were very low (~1%) for strains less than 0.1% and gradually increased to 10% for strains greater than 1%. The test results also indicated that frequency of loading does not significantly affect the moduli values, whereas the damping ratio values decreased with increasing frequency values. Based on the test results, shear modulus, G/G0 , versus cyclic shear strain amplitude, ξc , and damping ratio, D versus ξc , curves are proposed to describe the EPS geofoam nonlinear properties.