The objective of this study was to determine how the rice receptor-like cytoplasmic kinase OsBSR1 positively regulates broad-spectrum disease resistance. Our results showed that OsBSR1 is induced upon infection by Magnaporthe oryzae and enhances rice immunity by negatively regulating the enzymatic activity of the sulfite oxidase OsSO1, while also affecting its stability. In agreement with this finding, overexpression of OsSO1 suppressed the reactive oxygen species burst, callose deposition, and defense gene expression, resulting in reduced resistance to M. oryzae. These results uncover a novel kinase–oxidase regulatory module underlying the coordination of immune responses and redox homeostasis in rice.