Costunolide (COS) is a sesquiterpene lactone with anticancer properties. The present study investigated the anticancer effects of COS against the human colon (HCT116) and breast (MDA-MB-231-Luc) cancer cell lines. Inhibition of cell lines viability and IC50 of COS were assessed via an MTT assay. Furthermore, the apoptotic rate was detected by assessment of Bcl2-associated X (Bax) and B-cell lymphoma 2 (Bcl2) protein levels by flow cytometry. Xenograft mice model of HCT116 and MDA-MB-231-Luc were carried out to determine the effect of COS and its nanoparticles (COS-NPs). The results demonstrated that COS inhibited the viability of HCT116 and MDA-MB-231-Luc cells, with a half maximal inhibitory concentration value (IC50) of 39.92 µM and 100.57 µM, respectively. COS significantly increased Bax and decreased Bcl2 levels in treated cells. COS and COS-NPs, in combination with doxorubicin (DOX), significantly decreased the tumor growth of HCT116 and MDA-MB-231-Luc implants in mice. Furthermore, oral administration of COS and COS-NPs significantly decreased the viable cells and increased necrotic/apoptotic cells of HCT116 and MDA-MB-231-Luc implants. Interestingly, both COS and COS-NPs protected the cardiac muscles against DOX’s cardiotoxicity. The current results indicated the promising anticancer and cardiac muscles protection of COS and COS-NPs when administered with chemotherapy.