The study's objective was to evaluate using olive stone biochar (OSB), an inexpensive agro-waste product, to adsorb and remove inorganic mercury from Nile tilapia (Oreochromis niloticus) aquaculture systems. First, the OSB's adsorption capacity was evaluated by testing different concentrations: 1, 2 and 3 g L−1 for 0, 24, 48, 72 and 120 h. The concentration of Hg decreased with increasing OSB concentration up to 48 h, with the highest Hg adsorption rate observed at 2 g OSB L−1. An experimental study assessed the OSB's impact on Nile tilapia immune status and growth efficiency. The experiment used 180 Nile tilapia divided into four treatment groups (CONT, OSB, Hg and Hg +OSB), with three replicates per treatment. Fish were fed a normal basal diet twice daily for 60 d. The CONT group served as a control without any treatment. The OSB group was exposed to 2 g OSB L−1 of water, the Hg group was exposed to 0.084 mg Hg L−1, and the Hg +OSB group was exposed to 0.084 mg Hg L−1 with 2 g OSB L−1. Hg exposure caused liver and renal dysfunction and reduced growth performance, haematological indices, total antioxidant capacity, superoxide dismutase and non-specific immune parameters. Adding OSB to the pond water reduced these effects. In conclusion, adding 2 g OSB L−1 to aquarium water and changing it every 48 h mitigates the immunosuppressive effects of sub-chronic mercury toxicity and lowers mercury residues in fish muscle.