Background and Objectives: Tannin is a natural polymer that has recently been proposed for wood protection, but the structure of this polymer is not water resistant and can be easily hydrolyzed or leached out from the wood. The use of various aldehydes to synthesize tannin-based resins and therefore improve the properties of these resins has already been studied. Furfural is a natural aldehyde and is the raw material for the production of furfuryl alcohol, which is produced from the acidic hydrolysis of agricultural wastes, such as sugarcane. In this study, first, the effect of adding furfural on the tannin network features was investigated. Next, the ability of natural tannin/furfural resin was assessed to impregnate and modify poplar wood to improve its physical properties and hygroscopic behavior. Materials and Methods: Tannin-based resins were made with 20% m/m tannin and different ratios of furfural (2.5, 5, 10, 15, 20, 25, 40, and 50% of tannin dry weight) in different acidic conditions (pH 2, 4.5, 7, 9). For all formulations, the properties of resins were evaluated, including impregnation abilities, leaching resistance, viscosity, density and gel time. Optimal condition for making suitable resin to impregnate wood was selected by the complete factorial method using mini-tab software at a 95% confidence level. Then, the resin made under optimal condition was used to impregnate the poplar wood using the vacuum/pressure method. Heating was used to cure the resin and turn it into a polymer inside the wood. The physical properties of modified wood were measured according to ISO 13061 standard method. Results: Examination of resin properties showed that with an increase in furfural ratio, the resin density was increased. Moreover, the resin gel time and viscosity were increased, with increasing furfural ratio and acidity respectively. The highest leaching resistance of the resin was obtained with a resin composed of 50% furfural. Microscopic studies showed that res