The contamination of producer gas with tars from biomass gasification remains a significant challenge in the bioenergy industry and a critical barrier, limiting the commercial applications of biomass gasification. Non thermal and non-equilibrium plasma offers an unconventional and emerging technology for the effective reduction of problematic tars from gasification. In this study, we investigated plasma reforming of naphthalene as a two-ring tar model compound using a gliding arc discharge (GAD) reactor with/without steam. The influence on the plasma conversion of naphthalene based on the inlet naphthalene concentration, discharge power and steam-to-carbon ratio was examined to understand the effects of these operating parameters on the destruction of tar, gas selectivity/yield and energy efficiency. Adding H2O in the plasma process generates oxidative OH radicals, creating additional reaction routes for the step-wised oxidation of naphthalene and its fragments towards the CO, CO2 and water. The optimum ratio (2.0) of steam-to-carbon was identified to achieve the highest naphthalene conversion (84.8%), C2H2 yield (33.0%), total gas yield (72.2%) and energy efficiency (5.7 g/kWh). The effect of the amount of steam on the plasma reduction of tars was dependent on the balance between two opposite effects due to the presence of steam: positive effect of OH radicals and the negative effect of electron attachment on water molecules. Introducing an appropriate amount of steam to the plasma reduction of naphthalene also substantially minimized the formation of by-products and enhanced the carbon balance. Plausible reaction mechanisms for the plasma decomposition of naphthalene were proposed through a comprehensive analysis of gaseous and condensable products combined with plasma spectroscopic diagnostics.