Eco-friendly starch biofoams derived from natural rubber were fabricated in this study. To overcome their two main shortcomings, i.e., lack of flexibility and susceptibility to moisture, cotton fiber and concentrated natural rubber latex (CNRL) were incorporated into the biofoam products. Cotton-fiber-reinforced cassava starch biofoams were successfully prepared by compression molding using water as the solvent, a processing aid, and a blowing agent. The morphology and the physical, flexural, and thermal properties of the starch biofoams as a function of the CNRL content were investigated. The moisture adsorption capacity of the foam decreased with increasing CNRL content up to 5 phr (−73.4% and −41.78% at 0 and 100% RH, respectively). With increasing CNRL content, the hydrophobicity of the biofoam increased, as evidenced by contact angle measurements; this result suggested better moisture resistance and dimensional stability. The flexural properties of the biofoams were tuned by adjusting the CNRL content. With the addition of the CNRL content of 2.5 phr, the elongation of the biofoam clearly improved by 24%, with an acceptable decrease (−2.2%) in the bending modulus. Biodegradation by the soil burial test revealed that the degradation of the biofoam mainly proceeds by hydrolysis, and the addition of CNRL retards the degradation of the biofoam.