OBJECTIVE Interleukin (IL) 9 is a pleiotropic cytokine, and recent studies have demonstrated that IL-9 is associated with several cardiovascular diseases, via regulation of the inflammatory response. Doxorubicin (DOX) is known to induce severe cardiac injury and dysfunction by enhancing inflammation. This study aimed to investigate the role of IL-9 in DOX-induced cardiotoxicity. METHODS DOX was used to induce cardiac dysfunction and the expression of IL-9 in the murine cardiac tissues was measured. The mice were intraperitoneally injected with recombinant mouse IL-9 (rmIL-9) or anti-IL-9 neutralizing antibody (IL-9nAb) for investigating the effect of IL-9 on DOX-induced cardiac injury and dysfunction. The messenger ribonucleic acid (mRNA) expression levels of the pro-inflammatory cytokines were determined in each group by quantitative real-time polymerase chain reaction (RT-qPCR). The effect of rmIL-9 or IL-9nAb on DOX-induced apoptosis was determined both in vivo and vitro. RESULTS IL-9 levels significantly increased in the heart following DOX injection. Cardiac injury and dysfunction were induced by DOX, and treatment with IL-9nAb significantly alleviated DOX-induced injury, whereas rmIL-9 administration aggravated the cardiac damage. IL-9nAb decreased the expression of pro-inflammatory cytokines in the DOX-treated mice, while rmIL-9 administration increased the levels of pro-inflammatory cytokines. IL-9nAb reduced DOX-induced myocardial apoptosis, whereas rmIL-9 administration produced the opposite results. Additionally, IL-9nAb mitigated the DOX-induced apoptosis in H9C2 cells, while administration of rmIL-9 produced the opposite effect. CONCLUSIONS Our results demonstrated that IL-9 aggravated DOX-induced cardiac injury and dysfunction by promoting the inflammatory response and cardiomyocyte apoptosis.