This study aims to investigate the engineering properties (i.e., physical, chemical, and adhesion properties) of bitumen incorporated with RH-Warm Mix Asphalt (RH-WMA) additive. Base bitumen with a penetration grade of 80/100 was added with 3.0, 3.5, and 4.0% RH-WMA (by total weight of bitumen). The Response Surface Method (RSM) was used to determine the optimum values of the base bitumen and RH-WMA additive which meet the bitumen specification with penetration and softening point chosen as responses in the design layout. Three solutions of 3.04, 3.55 and 3.91% RH-WMA additive that were obtained from the RSM analysis were selected as the optimal values. Results of the physical tests show that adding RH-WMA increases the penetration and ductility values whilst simultaneously reducing the value for the softening point. Analysis of viscosity at various temperatures shows that RH-WMA additive lowers viscosity when the temperature was raised from 100 to 165 °C. The lower viscosity of the modified binders improves their workability. The result of the Fourier Transform Infrared (FTIR) test shows that there is no complex chemical reaction between the base bitumen and the RH-WMA. The surface free energy (SFE) test conducted on the base bitumen and the RH-WMA modified bitumen shows that the total SFE component of the base bitumen increases when higher amounts of RH-WMA was incorporated. The addition of 3.5% RH-WMA results in the highest SFE value of 24.98 mJ/m2. The increase in the total SFE suggests good adhesion between the bitumen and the aggregates.