Hepatocellular carcinoma is the second leading cause of cancer-related mortality worldwide. Lagerstroemia speciosa Pers. (Lythraceae) commonly known as Banaba has been used in different forms in traditional medicinal systems for treating various diseases which include diabetes and obesity. In this study, we investigated the cytotoxic potential of ethanolic Banaba leaf extract (EBLE) in HepG2 cells. The phytochemical analysis of EBLE was performed by HPTLC. HepG2 cells were treated with EBLE at 25, 50, 100, and 150 μg/mL concentrations, and cytotoxicity was evaluated by MTT assay. Oxidative stress was assessed by the evaluation of lipid peroxidation, superoxide dismutase, and reduced glutathione. Apoptosis-related morphology was investigated by acridine orange and ethidium bromide (AO/EB) dual staining. Mitochondrial membrane potential (ΔΨm) was evaluated by JC-1 staining. Apoptosis-related marker genes were evaluated by qPCR. HPTLC analysis confirmed the presence of corosolic acid (12.87 μg/mg), berberine (3.19 μg/mg), and gallic acid (2.94 μg/mg) in EBLE. EBLE treatments caused significant and concentration-dependent cytotoxicity and oxidative stress in HepG2 cells. Dual staining with AO/EB confirmed membrane distortion and nuclear chromatin condensation upon EBLE treatments. JC-I staining revealed the loss of ΔΨm. Furthermore, at a molecular level, EBLE treatments interfere with Bax/Bcl-2 homeostasis and induced the pro-apoptotic marker genes such as cytochrome c, Apaf-1, and caspases 9 and 3. EBLE treatments caused cytotoxicity in HepG2 cells, and this could be due to the induction of oxidative stress and apoptosis via the intrinsic or mitochondrial pathway.