The formation of nitrogen oxides (NO and N2O) during raw and demineralized biomass char combustion and the reduction of NO over biomass char were investigated. The biomass fuels were pine wood, straw, waste wood, bran, dried distillers grains with solubles (DDGS), and sunflower seed. Fixed-bed combustion experiments were performed at 800 °C in 10 vol % O2, while NO reduction experiments were conducted at temperatures from 800 to 900 °C and NO inlet concentrations from 400 to 1500 ppmv. The chars were characterized by means of inductively coupled plasma emission spectroscopy, Brunauer–Emmett–Teller, scanning electron microscopy and energy-dispersive X-ray, and X-ray photoelectron spectroscopy. The conversion of char-N to NO decreased with an increase in the initial char-N content, partly explained by the increased inherent conversion of char-N to N2O. The reduction of NO over char exhibited no correlation to the surface functionalities and content of nitrogen and oxygen at the investigated conditions. The NO reduction reactivity was strongly dominated by the content and association of ash-forming elements in the chars. The NO reduction reactivity of pine wood, waste wood, and straw chars correlated reasonably well with the (K + Ca)/C molar ratio, while the chars with a high phosphorous content, i.e., bran, DDGS, and sunflower seed chars, differed by showing a significantly lower reactivity. The inhibition effect of phosphorous on NO reduction reactivity was likely caused by the formation of less catalytically active potassium species (such as KPO3) in biomass char.