Nitrogen oxide (NOx) emissions from wildfires account for ~15% of the global total, inducing large fluctuations in the chemical production and loss rates of O3 and CH4 and thereby affecting Earth's radiative balance. NOx emissions from fires depend on fuel nitrogen (N) content, the relative contributions of flaming and smoldering combustion, and total biomass burned. Current understanding of the variability in these factors across biomes is limited by sparse observations. Here we use satellite-based measurements to study emission coefficients (ECs), a value proportional to NOx emitted per unit of biomass burned, from fires in African savannas. We show that ECs for NOx exhibit a pronounced seasonal cycle in woody savannas, with early-season ECs 20–40% above and late-season ECs 30–40% below the mean, while no cycle exists in nonwoody savannas. We discuss several possible mechanisms of the observed cycle including seasonal differences in fuel N content and modified combustion efficiency.