Chemical fertilizers have been widely used to improve rice production; however, their excessive use can have harmful environmental effects. Therefore, biostimulants are a sustainable option to promote rice yield and improve soil quality. This research focusses on the use of gamma-polyglutamic acid (γ-PGA) and Bacillus subtilis cells as biostimulants to improve rice growth and soil quality. The sand culture technique was performed to determine germination and growth of rice seedlings, and greenhouse experiments were conducted to evaluate the performance of rice yields. The soil quality was investigated by measuring physical and chemical characteristics. The results demonstrated that γ-PGA and B. subtilis cells were efficient biostimulants for germination by significantly increasing the seedling vigor index. γ-PGA considerably improved the growth parameters of 21-day-old rice seedlings by significantly increasing dry weight, total sugar, total free amino acids and total chlorophyll content compared to the control. In greenhouse experiments, γ-PGA had a positive influence on all physical characteristics and rice grain yield indicators compared to B. subtilis cells and controls. Furthermore, γ-PGA and B. subtilis cells had a stronger impact than controls on improving soil quality, and γ-PGA had a notable effect on soil physical properties rather than on their chemical properties. Based on these findings, γ-PGA outperformed B. subtilis cells as a natural biostimulant to increase rice productivity and improve the quality of paddy soil.