Clinical use of trastuzumab (TZM), has been widely associated with increased incidence of cardiotoxicity. Ocimum gratissimum Linn. is a household medicinal plant popularly used for treating inflammatory conditions. In this study, we investigated the abrogative potential of 100 mg/kg/day of the ethanol leaf extract of Ocimum gratissimum Linn. (OG) and its petroleum ether (PEOG), ethyl acetate (EAOG) and ethanol (EOG) fractions in TZM intoxicated Wistar rats for 7 days using anthropometric, biochemical, histopathological and immunohistochemical endpoints. In addition, secondary metabolite constituents in OG and its fractions were determined through Gas Chromatography-Mass Spectrometry (GC-MS). The study results showed that oral pretreatments with OG and OG fractions as well as the fixed dose valsartan-lisinopril (VAL-LSP) combination effectively ameliorated and restore nearly normal levels the TZM-altered plasma cardiac troponin I and antioxidant profile which were corroborated by histopathological and immunohistochemical findings as indicated by the inhibition of TZM-induced activation of caspases-3 and - 9 and profound upregulation of BCL-2 expression. Phytoscan of OG and its fractions showed the presence of thymol and in high amount. Overall, our findings revealed the cardioprotective potentials of OG, OG fractions and fixed dose VAL-LSP combination against TZM-induced cardiotoxicity which probably was mediated via abrogation of cardiomyocyte apoptosis and antioxidant mechanisms.