Assumed similarity is the tendency to assume that another person is similar to oneself. The present studies examined assumed similarity in intimate relationships regarding the HEXACO personality traits in Denmark (N = 93) and China (N = 236). Specifically, these studies hypothesized that people assume higher similarity with their intimate partners in Honesty-Humility and Openness to Experience compared to the other four HEXACO traits (Emotionality, Extraversion, Agreeableness, and Conscientiousness). Results from both studies indicate that assumed similarity was higher in Honesty-Humility compared to Emotionality, Extraversion, Agreeableness, and Conscientiousness. However, assumed similarity in Openness was higher compared to Emotionality and Conscientiousness only. Supplementary analyses indicate no cultural differences between Denmark and China in assumed similarity in Honesty-Humility and Openness.