Opportunism poses a significant challenge in business-to-business exchange relationships, particularly when companies aim for sustainable relationships. While trust is known to influence opportunism, its role across different stages of buyer-supplier relationships remains unclear in the existing literature. This study empirically investigates (1) the occurrence of opportunism before and after formal contract signing, specifically ex-ante and ex-post opportunism, and (2) the relational mechanisms for mitigating these types of opportunism. This study draws on transaction cost economics, contingency theory, and a process view of trust to explain the mechanism of how, when, and why trust influences opportunism throughout the different stages of a buyer-supplier relationship. An inductive theory-building approach through exploratory interviews methods was employed to address these theoretical issues. Data were collected through 25 semi-structured interviews conducted from April 2020 to November 2021 in both Japan and Vietnam. Our findings reveal the evolution of trust across different stages of buyer-supplier relationships: (1) in the initial stage, trust is formed by empirical evidence and relational assessment; (2) in the mature stage, trust is sustained by emotional connections and personal experiences. This dynamic interplay between trust and opportunism creates a powerful mechanism for mitigating opportunistic behavior. By understanding this process, managers can foster trust to safeguard against opportunism and build more collaborative and sustainable supplier relationships.