Aims <p>Dimethylarsinic acid (DMA) can induce straighthead disease in rice, resulting in substantial yield losses. DMA in rice originates from microbial arsenic (As) methylation, which is enhanced by the flooded conditions in paddy fields. The present study aims to develop strategies to alleviate straighthead disease via inhibiting arsenic methylation in paddy soil.</p> Methods <p>Field experiments were conducted to test the effects of calcium peroxide (CaO₂, a thermal stable inorganic peroxide that can release O₂ and H₂O₂ upon hydration) on microbial arsenic methylation and the incidence of straighthead disease in two paddy fields. CaO₂ was applied in single or split doses at the rice tillering stage.</p> Results <p>Applications of CaO₂ increased soil redox potential (Eh), reduced the abundance of functional genes involved in arsenic reduction (<i>arrA</i> and <i>arsC</i>) and methylation (<i>arsM</i>), and decreased both inorganic arsenic (iAs) and DMA concentrations in soil porewater. CaO₂ applications also significantly shifted the bacteria compositions, suppressing 11 and 212 amplicon sequence variants (ASVs) across 15 phyla in the two fields. CaO₂ applications significantly decreased the accumulation of iAs and DMA in rice husks and grains, alleviated straighthead disease, and increased rice yields compared to the control. Split application of CaO₂ produced a greater effect than single application.</p> Conclusions <p>CaO₂ application was effective at alleviating rice straighthead disease by suppressing microbial As methylation and subsequently decreasing DMA accumulation in rice.</p> Highlights <p>CaO₂ applications decreased the abundances of functional genes in microbial arsenic methylation in paddy soil.</p> <p>CaO₂ applications inhibited arsenic reductive release and methylation in paddy soil.</p> <p>CaO₂ applications decreased inorganic arsenic and dimethylarsenate accumulation in rice.</p> <p>CaO₂ applications alleviated straighthead disease of rice.</p>