Extruded polystyrene (XPS) has recently been used for construction such as in walls, and floors. When it is used for walls, axial load is inevitably applied along the length direction, raising concerns of collapse owing to buckling deformation. To address this, the buckling behavior of XPS should be appropriately characterized. However, such characterization has often been ignored because XPS has not conventionally been used as a structural material but solely as a thermal insulation material. In addition, the classical methods typically applied to analyze buckling behaviors are well-established; therefore, many researchers might not consider buckling analysis to be novel. However, as the use of XPS in construction increases, its buckling behaviors cannot be ignored, and few studies have investigated them to date. In this study, buckling tests of XPS were conducted using columns with various slenderness ratios, and the buckling stress–slenderness ratio was analyzed using the following three methods: the authors’ proposed method, Southwell’s method, and the modified Euler method. Independently of the buckling tests, short column compression and three-point bending tests were performed, and the buckling stress–slenderness ratio relationship was predicted using the properties obtained from these tests. Buckling stress could be effectively determined by these three methods across a wide range of slenderness ratios, whether elastic or inelastic buckling has occurred. Our proposed method was superior to the other two methods owing to its simplicity. In contrast, it was difficult to predict the buckling stress–slenderness ratio using the properties obtained from either the compression tests alone or three-point bending tests alone. However, the relationship could be appropriately determined using the properties obtained from both tests together.