This study investigated the potential protective effects of diphlorethohydroxycarmalol (DPHC), which was isolated from Ishige okamurae, on oxidative stress-induced myopathy both in vitro and in vivo. DPHC did not exhibit any toxic effects and exerted preventive actions against hydrogen peroxide (H2O2)-induced cell toxicity and scavenged intracellular reactive oxygen species in C2C12 myocytes. Furthermore, the DPHC markedly attenuated H2O2-induced myopathy in C2C12 cells by downregulating MuRF-1/MAFbx, which is a key factor in myopathy. The in vivo study revealed that the DPHC improved the endurance capacity and inhibited oxidative stress induced myopathy through the regulation of MuRF-1/MAFbx protein expressions via the NF-κB and p38 MAPK pathways in muscle tissue based on a H2O2-induced zebrafish model. Further, histological analysis indicated that the DPHC treatment significantly improved H2O2-induced muscle tissue loss and myopathy in vivo. Thus, our findings confirmed the viability of DPHC as a potential candidate for the treatment of oxidative stress-induced myopathy.