Compared to motivations for cross-sex friendship, little research has examined motivations for friendship between ex-partners after romantic relationship dissolution (i.e., post-relationship friendship; PRF). In Study 1, participants (N = 348) act nominated reasons for why someone might remain friends with an ex-partner. In Study 2, participants (N = 513) rated the importance of staying friends with an ex-partner for each reason given in Study 1 and completed the PID-5-BF and HEXACO to measure domains of clinically relevant and non-pathological personality. Principle component analysis identified seven categories of reasons for staying friends. Reasons that indicated that an ex-partner is reliable, trustworthy, and of sentimental value (i.e., reliability/sentimentality) were given the highest importance ratings whereas reasons that indicated that continued friendship was practical (i.e., pragmatism) were given the lowest ratings. Men rated pragmatism and sexual access reasons as more important than women did. Furthermore, antagonism scores on the PID-5-BF, and the Honesty–Humility and extraversion scores on the HEXACO predicted importance ratings for pragmatism and sexual access. Our findings are consistent with previous research and suggest that PRF may provide opportunity for ex-partners to exchange desirable resources (e.g., love, status, information, money, sex) after romantic relationship dissolution.