Sargassum, a type of brown seaweed, presents ecological, health, and economic challenges. However, its abundance also offers a unique opportunity to produce activated biochar (ABC). In this study, Sargassum biomass was harvested in Cancún, Quintana Roo, Mexico, and biochar (BC) was synthesized by pyrolyzing it at 500 °C. The biochar was subsequently activated using nitric acid (HNO₃) and characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The resulting activated biochar exhibited a nanostructured surface, which was utilized to develop an electrochemical genosensor. This was accomplished by immobilizing single-stranded DNA (ssDNA) onto a graphite disc electrode that had been modified with hydrolyzed collagen (HC) and 30% (w/v) ABC. This setup allowed for the detection of complementary DNA sequence hybridization (forming double-stranded DNA, dsDNA) through the technique of cyclic voltammetry. The Sargassum-derived ABC demonstrated exceptional properties, including a high surface area, porosity, and electrical conductivity qualities that are ideal for constructing electrochemical genosensors. Utilizing this type of biomaterial enhances biosensors, making ABC a crucial component in developing sensitive, portable analytical devices.