Doxorubicin (DOX) is well-known for its potent antitumor activity but limited by its multiple and serious adverse effects. A major adverse effect is acute cardiotoxicity; yet, its mechanism has not been elucidated. Fucoidan is a multifunctional and nontoxic polysaccharide that is widely studied because of its favorable biological activities and safety. Hence, we proposed that fucoidan may play a protective role in DOX-induced acute cardiotoxicity without causing additional side effects. Sprague-Dawley rats were injected intraperitoneally with a single high dose of DOX to induce acute cardiac injury. Fucoidan was administered orally before DOX injection and AG490, a JAK2 inhibitor, was applied to verify the participation of the JAK2/STAT3 pathway. In vitro, H9C2 cells were treated with the same drugs at different concentrations and intervention times. in vivo and in vitro results demonstrated that DOX administration induced myocardial damage accompanied by acceleratory apoptosis and deficient autophagy in heart tissues or cells, which could be significantly improved by fucoidan supplement. AG490 partly abolished the cardioprotective effects of fucoidan, suggesting the involvement of JAK2 signaling. Additionally, western blotting revealed DOX-induced JAK2/STAT3 pathway activation, which was enhanced by fucoidan and weaken by AG490. Hence, fucoidan exerted a favorable effect on DOX-induced cardiotoxicity by enhancing autophagy and suppressing apoptosis in a JAK2/STAT3-dependent manner, which may provide a promising and novel therapeutic strategy against negative chemotherapy-induced effects.