Doxorubicin is a chemotherapeutic medication commonly used to treat many types of cancers, but it has side effects including vomiting, rash, hair loss, and bone marrow suppression. The most dangerous side effects are cardiomyopathy, cardiofibrosis, and heart failure, as doxorubicin generates cytotoxicity and stops DNA replication. There is no treatment to block these side effects. We have developed a transgenic mouse line overexpressing the circular RNA circNlgn and shown that circNlgn is a mediator of doxorubicin-induced cardiofibrosis. Increased expression of circNlgn decreased cardiac function and induced cardiofibrosis by upregulating Gadd45b, Sema4C, and RAD50 and activating p38 and pJNK in circNlgn transgenic heart. Silencing circNlgn decreased the effects of doxorubicin on cardiac cell activities and prevented doxorubicin-induced expression of fibrosis-associated molecules. The protein (Nlgn173) translated by circNlgn could bind and activate H2AX, producing γH2AX, resulting in upregulation of IL-1b, IL-2Rb, IL-6, EGR1, and EGR3. We showed that silencing these molecules in the signaling pathway prevented doxorubicin-induced cardiomyocyte apoptosis, increased cardiomyocyte viability, decreased cardiac fibroblast proliferation, and inhibited collagen production. This mechanism may hold therapeutic implications for mitigating the side effects of doxorubicin therapy in cancer patients.