For the majority of people, two key non-kin figures form part of the central support clique that resides at the centre of their social network. These are the best friend and the romantic partner, and both play distinct roles which are of benefit to the individual concerned. However, while the romantic partner will always have been chosen in the context of the mating market, we do not know whether the selection of a best friend occurs within a similar market of competition and assessment. This study used real self-rated attribute data for participants and their best friends and romantic partners to explore: (1) whether best friendships operate within a mating market; (2) whether, once established, they show evidence for positive illusion, projection or competition; and (3) whether assortative mating is present. Further, we considered whether the sex of the best friend relative to the participant influences these results. We found that same-sex best friends have an acknowledged role linked to social connectedness and behaviour, that for same-sex best friends both male and female participants show evidence for homophily or projection rather than mate competition, that neither male nor female participants appear to view cross-sex best friends as potential mates, and that the evidence for ‘assortative mating’ is stronger within best friendships than romantic partnerships regardless of best friend sex. Our results imply that despite a culture of commitment and monogamy, male participants display behaviours within their romantic partnerships which suggest they are still active within the mating market. In contrast, for both sexes the best friendship is unaffected by the mating market and the stability and contentment that characterises best friendships is underpinned by a degree of similarity stronger than that within the romantic partnership.