Opisthorchis viverrini poses a substantial risk for cholangiocarcinoma (CCA) in Thailand. Despite praziquantel (PZQ) remains the primary treatment for opisthorchiasis, its association with adverse effects and potential CCA development during prolonged treatment, particularly in cases of reinfection and chronic infection, underscores the imperative for alternative herbal interventions with anthelmintic potential. In this context, a prior study suggested the inhibitory effects of Allium sativum L. (garlic) on various protozoa and helminths, prompting the investigation of its efficacy against O. viverrini in this study. Therefore, this study aimed to assess the efficacy of garlic against O. viverrini. O. viverrini adult worms were exposed to varying concentrations of garlic crude extract (20, 30, and 40 mg/ml). As comparators, another set of adult worms was treated with PZQ (20 mg/ml) and Roswell Park Memorial Institute 1640 medium, serving as the positive and negative control groups. The quantification of reactive oxygen species (ROS) as markers of oxidative stress was executed using 2',7'-Dichlorodihydrofluorescein diacetate staining. Morphological damage of O. viverrini adult worms were evaluated through scanning electron microscopy. Additionally, motility assessment was conducted at various exposure times (0, 30 minutes, 1, 2, 3, 6, 12, and 24 hours) by estimating relative motility values and survival index. The results revealed significantly elevated ROS levels and distinctive morphological damage, characterized by swelling of microvilli and papillae in the garlic-treated groups. In contrast, the positive control group exhibited minor morphological damage, while the negative control group did not display such alterations. The reduction in movement and increased mortality were observed in the groups treated with garlic, as evidenced by the RM and SI values, in comparison to both the positive and negative control groups. These findings suggest that garlic extract possesses potent anthelmintic properties against O. viverrini adult worms and holds promise as an alternative therapeutic avenue for Opisthorchiasis.