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Partial nitration‐anammox is a resource‐efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter‐acted by the emission of N2O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N2O formation and emission dynamics and to develop N2O mitigation strategies for a one‐stage partial nitritation‐anammox granular sludge reactor. Dynamic model calibration for such a full‐scale reactor was performed, applying a one‐dimensional biofilm model and including several N2O formation pathways. Simultaneous calibration of liquid phase concentrations and N2O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N2O emissions and qualitatively characterize the N2O dynamics, adjusting only seven parameter values. The model was validated with N2O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N2O formation pathway. Off‐gas recirculation as a potential N2O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of higher aeration energy consumption. [ABSTRACT FROM AUTHOR]