With the increasing population, a fire-protection of buildings has become necessary. Therefore, water-based indoor paints, which reduce the rate of the fire and offer the chance to intervene before the flames spread, have received attention; however, the performance and flame retardant property of these paints need to be improved. In this study, boric acid (H 3 BO 3 ), which is the product of boron (B), was used as an additive for a flame retardant in the prepared waterbased indoor paints. In order to investigate the effect of different weight ratios of H 3 BO 3 , the paints have been prepared with the addition of 5%, 10%, and 20% (w/w) of H 3 BO 3 . In addition, the synergistic effect of H 3 BO 3 and melamine (C 3 H 6 N 6 ) on the flame retarding performance of the prepared paints was also investigated. The physicochemical properties, flammability characteristics and thermal properties were analyzed by Fourier transform infrared spectroscopy (FTIR), limiting oxygen index (LOI), and thermogravimetric analysis (TGA), respectively. The results demonstrated that the characteristic peaks of H 3 BO 3 were observed in the prepared paints. When the flammability behavior of the paint samples were compared, the sample (P6) containing 20% (w/w) of H 3 BO 3 showed the highest fire resistance property. Moreover, the results of the thermogravimetric analysis demonstrated that the prepared paints containing H 3 BO 3 decomposes into B 2 O 3 and water, which suppresses the fire. It can be concluded that the prepared paint (P6) can be used as a suitable alternative for the water-based indoor paints.