<p>While existing literature primarily focuses on the impact of multimarket contact on inter-enterprise competition, it lacks analysis on the variation of firm market behavior under multimarket contact. This paper utilizes micro-survey data from Internet enterprises to empirically examine whether multimarket contact induces unethical behaviors such as “familiarity exploitation” and “price discrimination” among Internet enterprises. Based on data from a microsurvey of large Chinese Internet enterprises, the findings reveal that increased multimarket contact significantly increases unethical behavior. Furthermore, market size and resource similarity reinforce the impact of multimarket contact on unethical behavior. This conclusion holds true after robustness checks. Notably, the strategy of unethical market behavior exhibits action contagion among direct competitors, evolving into a collective immorality and the formation of “unspoken rules”. Finally, the paper presents relevant policy recommendations.</p>