Negotiations often involve information asymmetry, creating opportunities to use unethical tactics that trigger anger and frustration. This risk is particularly pronounced when negotiators come from different cultures with varying ethical standards. Yet existing research on when cultural differences in unethical negotiation behaviours emerge is inconsistent. This study applies Culture x Person x Situation (CuPS) model to examine the combined influence of cultural, personal and situational factors on perceived appropriateness of ethically ambiguous negotiation tactics (EANTs) in Face (China), Dignity (U.S.A.), and Honour (Mexico) cultures. A total of 552 participants completed online surveys, with negotiation situations manipulated to involve either one-off or future interactions. Stepwise regression analysis first revealed that negotiators with high fear of being exploited universally viewed EANTs as less inappropriate. More importantly, the negotiation situation moderated this relationship differently across cultures: in China, anticipating future interactions intensified the influence of personal fears; in the U.S.A., anticipating future interactions neutralised this influence; and in Mexico, the influence personal fear was the same in both future interactions and one-off negotiations. This study offers the first empirical evidence of the CuPS model in unethical negotiation research. The distinct cultural patterns challenge the conventional individualism-collectivism paradigm by emphasising unique ethical considerations in Honour (e.g., Mexico) collectivist cultures, which differ from those in East Asian (e.g., China) Face collectivist cultures. These findings broaden the scope of negotiation research and have important implications for understanding ethical judgements across cultural contexts.