To mitigate global warming and the serious problems incurred by the disposal of petroleum-based plastics, it is important to develop derivatives of biomass materials that can be used as substitutes. To overcome the lack of thermoplasticity of wood, a wet extrusion molding process for wood powder using a cellulose derivative, hydroxypropylmethyl cellulose (HPMC), had been developed. However, this material quickly reabsorbed water, swelled, and disintegrated in liquid. In the present study, a natural organic acid, citric acid, was added and kneaded together with the wood powder, the HPMC, and water. The resultant clay-like material was extruded into a tube-shaped material. The tube was air-dried and heated at 180 °C for 5 min to 30 min to allow crosslinking. By heating 1% citric acid for 30 min, the material avoided disintegrating in water for 60 min. The addition of 3% citric acid with 30 min crosslinking gave the material water resistance in water for 12 h. The degradability in the water was found to be controllable by changing the amount of citric acid and the heating time. This is a novel result because wood can be molded into a practical three-dimensional (3D) biomass composite material using this technology with natural substances without relying on petroleum-based plastics.