High-fiber-content composites made from rice husk (RH) (from 50 up to 80 wt%) as well as a recycled thermoplastic blend (rTPB) were fabricated using a two-step extrusion and hot/cold press molding technique. The temperature dependency of the thermal degradation and dynamic-mechanical behavior was investigated using thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The long-term water absorption and orthotropic swelling were analyzed following immersion in distilled, tap, and sea water for 15 weeks. Improvements in the thermal stability, storage, and loss modulus, as well as reductions in dimensional stability, were observed as the alkali content of the RH in the rTPB composites was increased. The composites immersed in sea water showed the lowest water absorption, followed by those in distilled and then tap water. The thickness dimension of the composite specimens exhibited the highest swelling values, followed by width and then length dimensions. The tensile strength and elastic modulus showed the maximum values at 70 wt% RH (21.2 MPa and 1.6 GPa, respectively). The surface morphology, interfacial adherence, and bonding between the matrix-fiber phases in the composites were characterized using a scanning electron microscope (SEM).