The effects of phosphorus (P) status on arsenate reductase gene (OsACR2.1) expression, arsenate reductase activity, hydrogen peroxide (H2O2) content, and arsenic (As) species in rice seedlings which were exposed to arsenate after −P or +P pretreatments were investigated in a series of hydroponic experiments. OsACR2.1 expression increased significantly with decreasing internal P concentrations; more than 2-fold and 10-fold increases were found after P starvation for 30 h and 14 days, respectively. OsACR2.1 expression exhibited a significant positive correlation with internal root H2O2 accumulation, which increased upon P starvation or exposure to H2O2 without P starvation. Characterization of internal and effluxed As species showed the predominant form of As was arsenate in P-starved rice root, which contrasted with the +P pretreated plants. Additionally, more As was effluxed from P-starved rice roots than from non-starved roots. In summary, an interesting relationship was observed between P-starvation induced H2O2 and OsACR2.1 gene expression. However, the up-regulation of OsACR2.1 did not increase arsenate reduction in P-starved rice seedlings when exposed to arsenate.