This study investigates the relationship between supplier power, affective trust, and supplier opportunism in buyer-supplier relationships. Drawing on transaction cost economics (TCE) and social exchange theory (SET), we propose that different types of supplier power influence the affective trust that purchasing agents place in their supplier agents, and that affective trust, in turn, negatively impacts both weak-form and strong-form supplier opportunism. We employ regression-based mediation analysis using the PROCESS module in SmartPLS 4 to test our hypotheses with survey data gathered from 548 purchasing agents in Vietnam. Our findings reveal that referent and legitimate powers positively influence affective trust, whereas coercive power has a negative effect. Surprisingly, reward power had no significant effect on affective trust. Furthermore, affective trust negatively impacts strong-form opportunism but positively impacts weak-form opportunism. These findings have important implications for academics and practitioners in managing buyer-supplier relationships.