The influence of water vapor on the production of nitric acid in the gas-phase HO₂ + NO reaction was determined at 298 K and 200 Torr using a high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer. The yield of HNO₃ was found to increase linearly with the increase of water concentration reaching an enhancement factor of about 8 at [H₂O] = 4 × 1017 molecules cm⁻³ (~50% relative humidity). A rate constant value k_1bw = 6 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ was derived for the reaction involving the HO₂*H₂O complex: HO₂*H₂O + NO → HNO₃ (1bw), assuming that the water enhancement is due to this reaction. k_1bw is approximately 40 times higher than the rate constant of the reaction HO₂ + NO → HNO₃ (1b), at the same temperature and pressure. The experimental findings are corroborated by density functional theory (DFT) calculations performed on the H₂O/HO₂/NO system. The significance of this result for atmospheric chemistry and chemical amplifier instruments is briefly discussed. An appendix containing a detailed consideration of the possible contribution from the surface reactions in our previous studies of the title reaction and in the present one is included.