Groups serve a variety of crucial functions, one of which is the provision of an identity and belief system that impart self‐referent information, thereby reducing self‐uncertainty. Entitative groups are more attractive for highly uncertain participants seeking groups for identification and self‐uncertainty reduction than less entitative groups. The purpose of the current study was to explore how self‐uncertainty impacts physiological arousal and stress responses. Using a mixed‐methods design (N = 123), we found that self‐uncertainty increased physiological arousal (measured via skin‐conductance level) and stress responses (measured via heart rate). Furthermore, we found that uncertainty‐activated physiological arousal and stress responses were decreased through identification with a high entitativity group. Our findings expand upon uncertainty identity theory by identifying physiological mechanisms that motivate uncertainty reduction. The purpose of this study was to explore how self‐uncertainty impacts arousal and stress. Our findings demonstrate how self‐uncertainty increased arousal and stress. Participants' arousal and stress decreased through identification with an entitative group. Results identify aversive mechanisms that motivate uncertainty reduction through identification. Our findings identify physiological mechanisms that motivate uncertainty reduction. [ABSTRACT FROM AUTHOR]