<p>Soil contamination with toxic metal(loid)s such as arsenic (As), cadmium (Cd), and lead (Pb) poses serious health risks by metal(loid) accumulation in crops. Biochar (BC) is a porous material that immobilizes metal(loid)s and modification of biochar further enhances its immobilization efficiency. Modified biochar (MBC) with iron phosphate can simultaneously immobilize As, Cd, and Pb in soil, as As is immobilized by iron oxides while Cd and Pb are immobilized by phosphate. Therefore, the purpose of this study was to evaluate and compare the effectiveness of BC and MBC in immobilizing metal(loid)s in contaminated soil and promoting the growth of lettuce. Both BC and MBC were applied to soils contaminated with either single or mixed metal(loid)s, including As, Cd and Pb. Shoot dry weight of lettuce grown in mixed metal(loid)-contaminated soil increased by 60.7% and 40.7% with BC and MBC, respectively, and root dry weight by 201% and 122% compared to the control. In mixed metal(loid)-contaminated soil, BC and MBC treatments reduced As uptake by 31.3% and 29.1% in shoots, 94.7% and 53.5% in roots, Cd uptake by 72.3% and 54.8% in shoots, 77.3% and 72.0% in roots, and Pb uptake by 88.3% and 86.2% in roots, respectively. In single As-contaminated soil, BC and MBC treatments increased bioavailable As, whereas in mixed metal(loid)-contaminated soil, the bioavailability of As, Cd, and Pb was reduced because of co-precipitation of As in solution with Cd or Pb. In addition, the phosphate and carbonate present in BC and MBC likely interacted with Cd and Pb, thereby reducing their competitive interactions with As and enhancing As immobilization in the mixed metal(loid)-contaminated soil. Therefore, BC and MBC effectively immobilized metal(loid)s and promoted lettuce growth in mixed metal(loid)-contaminated soils, making them suitable treatments for mitigating soil contamination and promoting plant growth. However, MBC did not show superior performance in metal(loid) immobilization compared to BC. </p>