<p>Oxytocin is a putative pharmacological target for social dysfunction. However, clinical trials of intranasal oxytocin have produced modest and inconsistent effects, motivating the search for naturalistic, non-pharmacological ways to recruit endogenous oxytocin. Here we show that a single session of live collegiate sports spectating is associated with endogenous oxytocin and interpersonal heart-rate (HR) synchrony, both of which were linked to psychological indices of social bonding among casual fans. Sixty adult spectators, including 21 female participants, provided repeated saliva samples for oxytocin and cortisol, continuous HR recordings, and ratings of enjoyment and unity before, during, and after a basketball game. Oxytocin dynamics were baseline dependent: individuals with low pre-game oxytocin showed clear increases, whereas those with high pre-game oxytocin maintained elevated concentrations; meanwhile, cortisol decreased and the oxytocin-to-cortisol ratio increased. HR synchrony also increased during and after the game. Notably, endogenous oxytocin levels showed a slight but positive association with the degree of HR synchrony, linking endocrine dynamics to interpersonal physiological coupling. In turn, higher oxytocin levels and stronger synchrony were associated with greater enjoyment, stronger unity, and intention to revisit, with perceived unity partially mediating links between physiological responses and experiential outcomes. These findings suggest that coordinated salivary oxytocin dynamics and physiological synchrony are candidate socio-physiological processes associated with social connection during live sports spectating. Live sports events may therefore serve as simple, scalable, non-pharmacological contexts in which oxytocin-related social experiences naturally arise in everyday life.</p>