The mitogen-activated protein kinase (MAPK) cascade is the most important mechanism in environmental responses and developmental processes in plants. The OsMAPK2 gene has been found to function in plant tolerance to diverse biotic/abiotic stresses. This paper presents evidence that OsMAPK2 (Oryza sativa MAP kinase gene 2) is responsive to P_i deficiency and involved in P_i homeostasis. We found that full-length expression of OsMAPK2 was up-regulated in both rice plants and cell culture in the absence of inorganic phosphate (P_i). The transgenic rice and Arabidopsis plants overexpressing OsMAPK2 showed affected root development and increased plant P_i content compared with wild-type plants. Overexpression of OsMAPK2 controlled the expression of several P_i starvation-responsive genes. Our results indicated that OsMAPK2 enables tolerance phosphate deficiency and is involved in P_i homeostasis.