This research explores the effects of chemical foaming agent and nanoclay on the long-term water absorption and cell morphology of wood–plastic nanocomposites. To meet this objective, the nanoclay, high-density polyethylene, and coupling agent were compounded and then granulated and were mixed with wood flour and blowing agent in a twin-screw extruder. Consequently, foaming process was completed in injection stage. The amount of wood flour was fixed at 50 wt% for all the samples. For nanoclay and chemical blowing agent, different levels of 0, 2 and 4 per hundred resins (phr) and 0, 0.5, 1 and 1.5 phr were considered, respectively. The long-term water absorptions of samples were evaluated by immersing them in water for several weeks, and water diffusion coefficients were also calculated by evaluating the water absorption isotherms. Results indicated that the water absorption, cell size, and average cell density of composites increased with increase in chemical foaming agent loadings. However, by the addition of nanoclay to wood–plastic composite foams, the water absorption and cell size decreased and their cell density increased. Scanning electron microscopy confirmed that the chemical foaming agent and nanoclay contents had significant influence on density reduction in foamed composites. X-Ray diffraction patterns revealed that the nanocomposites formed were intercalated. The mechanism of water absorption of the foamed wood–plastic nanocomposites under study followed the kinetics of a Fickian diffusion process.