Nitrogen and water are two important factors influencing GHG (primarily CO₂ - carbon dioxide; CH₄ – methane, and N₂O – nitrous oxide) fluxes in semiarid grasslands. However, the interactive effects of nitrogen and water on GHG fluxes remain elusive. A 3-year (2010 − 2012) manipulative experiment was conducted to investigate the individual and interactive effects of nitrogen and water additions on GHG fluxes during growing seasons (May to September) in a semiarid grassland in Northern China. Accumulated throughout growing seasons, nitrogen input stimulated CO₂ uptake by 3.3 ± 1.0 g C m^− 2 (g N)^− 1, enhanced N₂O emission by 1.2 ± 0.3 mg N m^− 2 (g N)^− 1, and decreased CH₄ uptake by 5.2 ± 0.9 mg N m^− 2 (g N)^− 1; water amendment stimulated CO₂ uptake by 0.2 ± 0.1 g C m^− 2 (mm H₂O)^− 1 and N₂O emission by 0.2 ± 0.02 mg N m^− 2 (mm H₂O)^− 1, decreased CH₄ uptake by 0.3 ± 0.1 mg C m^− 2 (mm H₂O)^− 1. A synergistic effect between nitrogen and water was found on N₂O flux in normal year while the additive effects of nitrogen and water additions were found on CH₄ and CO₂ uptakes during all experiment years, and on N₂O emission in dry years. The nitrogen addition had stronger impacts than water amendment on stimulating CH₄ uptake in the normal year, while water was the dominant factor affecting CH₄ uptake in dry years. For N₂O emission, the N-stimulating impact was stronger in un-watered than in watered plots, and the water-stimulating impact was stronger in non-fertilized than in fertilized treatments in dry years. The interactive impacts of nitrogen and water additions on GHG fluxes advance our understanding of GHG fluxes in responses to multiple environmental factors. This data source could be valuable for validating ecosystem models in simulating GHG fluxes in a multiple factors environment.