<p>Mercury (Hg) contamination associated with artisanal gold mining represents a major environmental and public health concern in South America, particularly in fish-dependent Amazonian communities. In this study, we investigated the potential of nanobiochar and thiol-modified nanobiochar to reduce Hg bioavailability in an Amazonian soil contaminated by illegal gold mining. Pristine nanobiochar (NB) and thiol-functionalized nanobiochar (NBS) were produced from açaí (<i>Euterpe oleracea</i>) processing residues and applied to soil under controlled incubation conditions. Changes in Hg partitioning were assessed using sequential chemical extractions, allowing the evaluation of exchangeable, reducible, oxidizable, and residual Hg fractions. Both NB and NBS decreased the exchangeable Hg fraction, while increasing Hg associated with more stable soil compartments. The application of NBS reduced Hg availability of up to 17%, with enhanced Hg retention relative to NB. This behavior was attributed to differences in surface chemistry, including oxygen-containing functional groups (carboxyl, and phenolic) in NB and the presence of sulfur-containing groups in NBS, which promoted stronger Hg binding through covalent interactions. Although the low Fe content of the soil limited Hg stabilization in the reducible fraction, thiol modification enhanced Hg immobilization overall. These results demonstrate that thiol-modified nanobiochar can effectively alter Hg partitioning toward less bioavailable forms, highlighting its potential as a soil amendment for mitigating Hg risks in contaminated tropical environments.</p>