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Steel slag, a by-product of the steel industry, contains high amounts of active iron oxide and silica which can act as an oxidizing agent in agricultural soils. Biochar is a rich source of carbon, and the combined application of biochar and steel slag is assumed to have positive impacts on soil properties as well as plant growth, which are yet to be validated scientifically. We conducted a field experiment for two rice paddies (early and late paddy) to determine the individual and combined effects of steel slag and biochar amendments on CO2, CH4, and N2O emission, and rice productivity in a subtropical paddy field of China. The amendments did not significantly affect rice yield. It was observed that CO2 was the main greenhouse gas emitted from all treatments of both paddies. Steel slag decreased the cumulative CO2 flux in the late paddy. Biochar as well as steel slag + biochar treatment decreased the cumulative CO2 flux in the late paddy and for the complete year (early and late paddy), while steel slag + biochar treatment also decreased the cumulative CH4 flux in the early paddy. The biochar, and steel slag + biochar amendments decreased the global warming potential (GWP). Interestingly, the cumulative annual GWP was lower for the biochar (55,422 kg CO2-eq ha(-1)), and steel slag + biochar (53,965 kg CO2-eq ha(-1)) treatments than the control (68,962 kg CO2-eq ha(-1)). Total GWP per unit yield was lower for the combined application of steel slag + biochar (8951 kg CO2-eq Mg-1 yield) compared to the control (12,805 kg CO2-eq Mg-1 yield). This study suggested that the combined application of steel slag and biochar could be an effective long-term strategy to reduce greenhouse gases emission from paddies without any detrimental effect on the yield.