Worldwide, snakebites have serious implications for human health. The administration of antivenom is the official treatment used to reverse the toxic activities of envenomation. However, this therapy is not efficient to treat the local effects, leading to the amputation or deformity of affected limbs. As such, alternative treatments are needed. Here, we analyze the ability of a polysaccharide from the green marine alga Gayralia oxysperma (Go3) to inhibit the effects of venom from Bothrops jararaca and Lachesis muta. B. jararaca or L. muta venoms were incubated together with sulfated heterorhamnans from Go3, and the in vitro (coagulation, proteolytic, and hemolytic) and in vivo (hemorrhagic, myotoxic, edematogenic, and lethal) activities of venoms were assessed. Additionally, Go3 was injected before and after the injection of venoms, and the toxic activities were further tested. When incubated with the venoms, Go3 inhibited all activities, though results varied with different potencies. Moreover, Go3 neutralized hemorrhagic, myotoxic, and edematogenic activities when injected before or after injection with B. jararaca and L. muta venom. Go3 also blocked the coagulation of plasma in mice caused by the venoms in an ex vivo test. Therefore, Go3 has the potential to be used as antivenom for B. jararaca and L. muta bites, notably exhibiting higher efficacy on L. muta venom.