The effect of triphenyl phosphate (TPP) retardant inhibition on flame propagation over the horizontal surface of polymethyl methacrylate (PMMA) has been studied experimentally and numerically. Regarding the flame spread behavior over the surface of PMMA (pure and inhibited by TPP), the following parameters were measured: thermal decomposition with TG / DTG analyzer, the flame spread rate, the pyrolysis zone length, the mass loss rate and spatial distribution of temperature by thermocouples and species concentration in the gas-phase flame by probing mass spectrometry. The previously developed coupled heat and mass transfer mathematical model describing the feedback interaction between flame and solid fuel, as well as volatilization of pyrolysis products, was modified to resolve the effect of TPP on flame spread by introducing the correcting factor of the gas-phase combustion reaction rate relating to the inhibitor concentration in the solid material. Good agreement between the measured and calculated flame spread parameters (flame spread velocity, mass burn-out rate, pyrolysis zone length), as well as a detailed flame structure (gas phase temperature and species concentration), has been obtained for pure PMMA and PMMA + 10%TPP. It has been shown that the proposed approach describes a satisfactory retardant effect of TPP on the flame spread over PMMA surface by inhibiting the gas-phase combustion reaction.