The horse-rider relationship is fundamental to ethical equestrianism wherein equine health and welfare are prioritized as core dimensions of sporting success. Equestrianism represents a unique and important form of interspecies activity in which relationships are commonly idealized as central to sporting performance but have been largely unexplored in the sport psychology literature. Horse-rider relationships warrant particular consideration in the elite sporting context, given the tension between constructions of “partnership” between horse and rider, and the pragmatic pressures of elite sport on horse and rider and their relationship. The current study examined the link between sporting performance and the horse-rider relationship in an elite equestrian sporting context. Thirty-six international elite riders from eight countries and six equestrian disciplines participated in a single in-depth interview. A social constructionist, grounded theory methodology was used to analyze this data. The horse-rider relationship was positioned in three different ways in relation to elite sporting outcomes: as pivotal to success; non-essential to success; or as antithetical to success. Participants shifted between these positions, expressing nuanced, ambivalent attitudes that reflected their sporting discipline and their personal orientation to equestrianism. Competitive success was also defined in fluid terms, with participants differentiating between intrinsic and extrinsic markers of success. These findings suggest a complex and multifaceted connection between interspecies performance and relationships in elite sport. Where strong horse-rider relationships are antithetical to performance, a threat to the welfare and ethics of equestrian sport exists. Relevant sporting governing bodies must attend to this problem to ensure the centrality of animal welfare, wellbeing, and performance longevity to equestrian sports.