Social support plays a crucial role in promoting positive outcomes for athletes, including reduced stress and enhanced well-being. However, the dynamics of social support processes in sport are complex and multifaceted, and existing research lacks insight into actual social support interactions among teammates. Additionally, the physiological arousal shared between athletes during discussions about sport-related stressors remains poorly understood. This study aimed to observe social support behaviors exhibited by athletes during discussions of sport-related stressors while continuously recording athletes’ physiological arousal, specifically focusing on co-regulation of heart rate variability. Laboratory-based conversations were conducted with 46 dyads of university athletes (mean age = 20.2 years, SD = 1.9). Video recordings, self-report data, and continuous recordings of heart rate variability were analyzed. Findings revealed that athlete dyads commonly engaged in co-rumination, collectively emphasizing negative feelings associated with their stressors. Post-conversation stress ratings were lower than pre-conversation stress ratings, suggesting that discussing stressors with a teammate may alleviate athletes' stress. Yet analyses indicated a positive association between co-rumination and perceived stress. Moreover, co-rumination related with co-dysregulated physiological arousal—a process in which partners’ physiological arousal is linked and mutually amplifying and may be related with regulatory failures. These results highlight the significance of considering co-rumination and its association with co-dysregulated physiological arousal in social support interactions. Future research should explore the impact of co-rumination on stress perception and physiological responses among athletes.