Testosterone reactivity has been conceptualized as a marker of social submission at low levels and social dominance at high levels. However, hormonal fluctuations in response to romantic partners remain largely unknown. Towards this end, 88 couples (N = 176) discussed an emotional video. Prior to the conversation, one member of the dyad (the “agent”) was instructed to regulate affective displays in a specific way (express or suppress). The other dyad member (the “partner”) was given no special instruction and was unaware of regulation instructions given to the agent. Agents who regulated affective displays were expected to exhibit decreased testosterone from baseline because they were prevented from tuning their emotional responses to their partners. Furthermore, we expected declines in testosterone would be moderated by partners’ authoritativeness: People would be particularly submissive to more dominant partners. Predictions were supported for females and partially supported for males. Agents exhibited decreases in testosterone from baseline relative to partners. For females, this main effect was moderated by partners’ trait-level authoritativeness: Females interacting with partners higher in authority exhibited larger decreases in testosterone when instructed to restrict their emotion regulation strategies. This research is the first to document testosterone reactivity in existing romantic relationships and underscores the importance of taking into account social and relational contexts when examining hormonal regulation.