Music has been argued to contribute to well-being in multiple ways, through its links to identity, social relationships, emotion, and memory. We investigated the phenomenon of “couple-defining songs (CDSs),” in which members of a couple come to jointly identify their relationship with a particular song. Two hundred participants who were currently in a romantic relationship, diverse in age and relationship length and status, reported whether they had a CDS. Those who reported a CDS described its origins and meaning, and any memories and emotions elicited by thinking about their song. In addition, participants completed measures of music appreciation and relationship intimacy. We found that CDSs were common, relatively unique to romantic relationships, and associated with higher music appreciation and higher intimacy. CDSs tended to be acquired early in relationships, and they cued positive emotions and specific memories. These findings suggest that CDSs represent a common and understudied phenomenon. We propose that the multifaceted nature of music may contribute to the prevalence of CDSs in intimate relationships.