<p>Physiological synchronization has been observed in empathy. However, the specific role of breathing synchronization in emotional states remains unexplored. Here, we found that synchronized breathing that matches the observation of another person’s joyful facial expression increased empathy and perceived familiarity. In this study, we measured emotional arousal, familiarity, empathy, and favorability to participants while presenting face stimuli exhibiting six basic emotional expressions (i.e., neutral, joy, surprise, anger, fear, and sadness) under three conditions (i.e., static image, image with asynchronous breathing, and image with synchronized breathing in relation to the participant). In the synchronized condition, a high-precision millimeter-wave radar system was used to capture participants’ real-time breathing patterns, which allowed non-perceptible respiratory monitoring. Comparisons between emotional expressions revealed that the synchronized breathing condition with joyful stimuli was associated with the highest empathy and familiarity, which suggested that a physiological mechanism underlying shared breathing patterns facilitated emotional connection with joyful expressions. These findings offer insights into the embodied basis of positive emotional sharing underlying synchronized breathing during social activities, such as group singing and shared laughter.</p>