The conversion of combined nitrogen (ammonia, nitrate, organic nitrogen) to dinitrogen (N2) in marine sediments, an important link in the global nitrogen cycle, is traditionally assumed to take place only via the coupled bacterial nitrification-denitrification process. We provide field and laboratory evidence that N2 can also be produced by the oxidation of NH3 and organic-N with MnO2 in air. The reduced manganese formed in this reaction readily reacts with O2, generating reactive Mn(III, IV) species to continue the oxidation of NH3 and organic-N to N2. Free energy calculations indicate that these two reactions are more favorable as a couple than the oxidation of organic matter by O2 alone. We also provide field evidence consistent with the reduction of NO-3 to N2 by dissolved Mn2+. These two reactions involving nitrogen and manganese species can take place in the presence and absence of O2, respectively. Our field evidence suggests that the oxidation of NH3 and organic-N to N2 by MnO2 in the presence of O2 can outcompete the oxidation of NH3 to NO-3 in Mn-rich continental margin sediments and thereby short-circuit the nitrification/denitrification process. The MnO2 catalyzed reaction may account for up to 90% of the N2 formation in continental margin sediments, the most important N2 producing environments in the marine N cycle. The oxidation of NH3 and organic N by MnO2 in the presence of O2 can explain why N2 can form in oxic sediments; it can also explain why denitrification rates measured by acetylene inhibition and labeled tracers can give lower estimates than direct measurements of N2 production.