The possibility for anaerobic NH4+ oxidation and N2 formation was explored in a Mn oxide-rich continental basin sediment from Skagerrak. The surface sediment contained 2.9 weight-% Mn(IV), and reactive Mn oxide persisted to ≥10 cm depth. Microbial Mn reduction completely dominated anaerobic carbon oxidation, whereas neither Fe reduction nor sulfate reduction were significant. Accumulation rates of soluble NH4+ during anoxic incubations scaled with Mn reduction rates and did not indicate any substantial oxidation of NH4+. No sustained production of 15N-labelled N2 from added 15NH4+ was detectable during the four-day incubations, which constrains the rate of NH4+ conversion to N2 to <2% of the NH4+ production rate. Traces of 15N-labelled N2 accumulated initially, and this transient N2 production was possibly related to brief coupled nitrification/denitrification resulting from sediment handling. Oxidation of NH4+ to NO3− was also insignificant as there was no accumulation of NO3− during the incubations and added 15NO3− was rapidly consumed with N2 as a major product. Although the oxidation of NH4+ with Mn oxide is thermodynamically favorable, our results demonstrate that such oxidation was insignificant and that NH4+ can be considered the end product of nitrogen mineralization in this anoxic Mn oxide-rich sediment.