(1) Background: Nitrogen (N) fertilization on drained tropical peatland will likely stimulate peat decomposition and mineralization, enhancing N2O emission from the peat soil. (2) Methods: A field experiment was conducted to quantify the N2O emissions from soil in an oil palm plantation (Elaeis guineensis Jacq.) located in a tropical peatland in Sarawak, Malaysia, under different rates of N fertilizers. The study was conducted from January 2010 to December 2013 and resumed from January 2016 to December 2017. Nitrous oxide (N2O) flux was measured every month using a closed chamber method for four different N rates; control-without N (T1), 31.1 kg N ha(-1) yr(-1) (T2), 62.2 kg N ha(-1) yr(-1) (T3), and 124.3 kg N ha(-1) yr(-1) (T4); (3) Results: Application of the N fertilizer significantly increased annual cumulative N2O emissions for T4 only in the years 2010 (p = 0.017), 2011 (p = 0.012), 2012 (p = 0.007), and 2016 (p = 0.048). The highest average annual cumulative N2O emissions were recorded for T4 (41.5 +/- 28.7 kg N ha(-1) yr(-1)), followed by T3 (35.1 +/- 25.7 kg N ha(-1) yr(-1)), T1 (25.2 +/- 17.8 kg N ha(-1) yr(-1)), and T2 (25.1 +/- 15.4 kg N ha(-1) yr(-1)), indicating that the N rates of 62.2 kg N ha(-1) yr(-1) and 124.3 kg N ha(-1) yr(-1) increased the average annual cumulative N2O emissions by 39% and 65%, respectively, as compared to the control. The N fertilization had no significant effect on annual oil palm yield (p = 0.994). Alternating between low (deeper than -60 cm) and high groundwater level (GWL) (shallower than -60 cm) enhanced nitrification during low GWL, further supplying NO3- for denitrification in the high GWL, and contributing to higher N2O emissions in high GWL. The emissions of N2O ranged from 17 mu g N m(-2) hr(-1) to 2447 mu g N m(-2) hr(-1) and decreased when the water-filled pore space (WFPS) was between 70% and 96%, suggesting the occurrence of complete denitrification. A positive correlation between N2O emissions and NO3- at 70-96% WFPS indicated that denitrification increased with increased NO3- availability. Based on their standardized regression coefficients, the effect of GWL on N2O emissions increased with increased N rate (p < 0.001). Furthermore, it was found that annual oil palm yields negatively correlated with annual N2O emission and NO3- for all treatments. Both nitrification and denitrification increased with increased N availability, making both processes important sources of N2O in oil palm cultivation on tropical peatland.; and (4) Conclusions: To improve understanding of N2O mitigation strategies, further studies should consider plant N uptake on N2O emissions, at least until the completion of the planting.