The potential health risks associated with high levels of heavy metals, particularly mercury, encompass a wide range of often irreversible toxic effects. Mercury pollution resulting from industrial activities has serious repercussions on the environment, human health and ecosystems. In recent years, much research has focused on the use of bio-based materials for wastewater treatment. This study explores the use of biochars derived from the pyrolysis of biomass—as adsorbents to remove Hg(II) mercury from aqueous solutions. Biochars are derived from the pyrolysis of alginate extraction residue from the brown seaweed “Sargassum muticum”, namely R350/60, R350/90 and R350/120, and represent cost-effective and environmentally-friendly adsorbents. Characterization involved thermogravimetric analysis (TGA), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (SEM–EDS), N2 physisorption, Fourier transform infrared (FTIR) and Raman spectroscopy. The results revealed greater biosorption of Hg(II) using biochar produced at low-temperature treatments. Langmuir and Freundlich adsorption isotherms were used to fit the experimental adsorption data. The Langmuir equilibrium model was used to validate the study's remarkable adsorption capacity (470.53 mg/g) of biochar made from pyrolyzed residue at 350 °C. A perfect fit with the pseudo-second-order model was verified by kinetic analysis, demonstrating effective mercury removal. The functional groups on the surface of the biochar actively promoted the adsorption process. These outcomes highlight its potential as an inexpensive, environmentally beneficial mercury removal method.