Chromium (Cr) pollution in soil is a global problem owing to its wide industrial use. The mobility, toxicity, and crop uptake of Cr depends on its valence state. Cr(VI) is highly mobile and toxic whereas Cr(III) is generally considered immobile and less toxic. We performed a pot experiment to investigate the combined effects of rice straw-derived biochar and water management on transformation of Cr and its uptake by rice in contaminated soils. The main plots had water management treatments of alternating wetting and drying (AWD) and continuous flooding (CF), and the subplots had three levels of straw biochar (0, 5, and 10 g kg^-1). The results showed that water management and the addition of biochar had a significant effect on the dynamics of soil redox potential (Eh), pH, dissolved organic carbon (DOC), and Fe(II) concentration. As these parameters are important factors affecting Cr transformation in paddy soils, the dynamics of the Cr(III) and Cr(VI) concentrations were clearly different under different treatments. The highest reduction of Cr(VI) was observed in the treatment with CF water management in combination with 10 g kg^-1 of biochar amendment, which resulted in a 62% reduction of Cr(VI) to Cr(III) in soil. The alterations in the oxidation state of Cr greatly affected its accumulation in the rice grains. The CF combined with 10 g kg^-1 of biochar treatment, caused the Cr concentration in rice grains to be 66.2% lower compared with that of the unamended control under AWD water management. Possibly owing to the reduction in phytotoxic effects of Cr(VI), the combined treatment showed an improvement in rice grain weight. In conclusion, the combination of 10 g kg^-1 of biochar amendment and CF water management may potentially be used in Cr-contaminated soil to mitigate the impacts of Cr contamination on rice production.