In this research, recycled plastic wastes and unexploited natural rice straw (RS) are used as raw materials to produce a new type of wood plastic composite. Recycled high-impact polystyrene (HIPS) and mechanically fiberized RS (without any chemicals) were mixed together twice through a single extruder followed by a press process using a flat press to produce an eco-friendly synthetic wood called rice straw plastic composite (RSPC). Samples with different densities (950, 1100, and 1250 kg/m3) and different RS fiber ratios (40, 50, and 60 wt%) were produced. The effects of the RS fiber ratio and the RSPC density on flexural properties and water uptake characteristics were investigated. It was found that the modulus of elasticity and modulus of rupture reached maximum values of 4917 MPa and 25.2 MPa at 1250 kg/m3 of composite density with 60% and 50% fiber ratio, respectively. The highest values for water absorption and thickness swelling were recorded at 1250 kg/m3 composite density and 60% fiber content to be 3.96% and 2.64%, respectively. Comparison of the results with other RSPCs indicated the feasibility of fabrication of the investigated RSPC. Moreover, the use of additives like coupling agents could further enhance the properties of the produced RSPC.