Sand as a building material is pretty common in engineering constructions such as slopes and foundation pits. The great instability is generally generated because of the lack of cohesion inside and such instability is more serious during rains or when subjected to excessive external loads. This paper is aimed to study a combination of polyvinyl acetate polymer and sisal fiber as reinforcement materials. The focus of this study is to determine the effects of polymer content, fiber content, and dry sand density on the improvement in mechanical behavior of reinforced poorly graded sand. A series of direct shear, unconfined compression, and tensile tests on the reinforced sand have been conducted. The results suggest that the reinforcement effect increases with dry sand density up to 1.55 g/cm³ and then levels off. For fixed dry sand density, the strength of the improved sand enhances with the augment of polymer and plant fiber contents. At maximum contents, the reinforced sand can have 480 kPa of shear tensile strength, 1,276 kPa of unconfined compressive strength, and 240 kPa of tensile strength. The reinforcement mechanisms are revealed by Scanning Electron Microscope images. Polymer forms firm the polymer–soil matrices that enhance the fiber–sand interactions to mobilize tensile stresses. The combined use of polymer and fiber is preferred in engineering practices because of its zero negative impacts on the environment.