<p>Soil contamination with arsenic (As) and heat stress cause decline in crop production and human health risks on a global scale. The current study was a controlled pot experiment. It was undertaken to determine the usefulness of biochar in alleviating phytotoxicity of As and heat stress in quinoa. The health risks for adults and children associated with consumption of As-tainted grains were also assessed. Quinoa plants were grown on As contaminated (20&#xa0;mg&#xa0;kg⁻¹) soil under ambient temperature (30/12 ℃ day/night), and heat stress (5 ℃ higher than ambient temperature) supplemented with 0% and 2% biochar. The combination of As with heat stress caused the highest decline in shoot/root dry weights (46/47%), stomatal conductance (48%), and leaf relative water contents (30%) as compared to the control. Under heat stress, As accumulation in shoot, root and grains was enhanced. Arsenic and heat stress together caused the generation of H₂O₂ and disruption of the cell membrane. Biochar application boosted the growth of quinoa by improving the physiological characteristics of plants. Oxidative stress was ameliorated under biochar application, primarily due to enhanced activity of antioxidant enzymes (SOD, CAT, POD). Non-cancerous and cancerous risks to adults and children through consumption of As contaminated quinoa grains were reduced under biochar amendment. Furthermore, phytostabilization potential for As was increased under biochar application. Thus, the application of cotton shell biochar is a suitable approach for decreasing the phytotoxicity and human health risks caused by As under heat stress conditions.</p>