This study investigated the determinants of exchange anxiety in close relationships—generalized worries that one’s partner will replace him/her with a more suitable person. We hypothesized that exchange anxiety would increase when individuals compare themselves with others who are more competent (experiencing upward social comparisons), especially in environments where they cannot easily find alternative relationships (environments with low relational mobility). By experimentally manipulating the type of social comparison, the results from Study 1 revealed that undergraduates (n＝299) living in environments with low relational mobility felt stronger exchange anxiety when they experienced upward social comparison than downward social comparison. In Study 2, an online survey was conducted with a sample of adults living in either urban or rural areas (n＝1000). The results showed that the frequency of upward social comparison was positively associated with exchange anxiety and that this tendency was moderated by the combined effect of relational mobility and trait self-esteem. These results suggest that the characteristics of one’s interpersonal environments affect the impact of the perceived risk of being replaced on exchange anxiety in one’s close relationships.