This study aimed to investigate the effects of biochar on soil nutrient dynamics and rice productivity under different irrigation regimes. A pot experiment of rice straw biochar application (0, CK; 2%, C1; 4%, C2; 6%, C3) combined with three types of irrigation regimes (continuous flooding, CF; intermittent irrigation, II; saturated moisture, SM) was conducted. The N content of the biochar increased by 25.27–32.26% after application, while the C (12.09–19.06%), P (25.58–32.56%), and K (89.74–91.67%) contents in the biochar decreased. The total N content in the biochar-amended soil peaked at the heading stage of the first year. Compared with CK, the total N increased by 14.47–34.87%, 17.20–49.04%, and 28.85–42.31% under CF, II, and SM, respectively. The available P content in the biochar-amended soil was highest at the jointing stage under CF conditions, whereas it was highest at the heading stage under SM. In addition, the maximum increase in available P was 19.34, 14.32, and 16.10 mg kg−1 under CF, II, and SM, respectively. The increase in available K content induced by biochar was the most significant and reached a maximum (13.1, 11.5, and 10.5 fold under CF, II, and SM, respectively) at the jointing stage. Biochar had the better potential for yield enhancement (10.78–35.32%) under II, mainly by increasing the effective panicles (0–6.6%) and spikelets per panicle (17.57–24.91%) and dry matter productivity (8.66–27.65%). The increase in P and K in the soil may have directly originated from the biochar, while N was not. Continuous flooding promoted the release of P and K from the biochar, whereas biochar plus intermittent irrigation benefitted rice production.