Understanding how information about threats in the environment is shared and transmitted between individuals is crucial for explaining adaptive, survival-related behavior in humans and other animals, and for developing treatments for phobias and other anxiety disorders. Research across species has shown that observing a conspecific’s, a “demonstrator’s”, threat responses causes strong and persistent threat memories in the “observer”. Here, we examined if physiological synchrony between demonstrator and observer can serve to predict the strength of observationally acquired conditioned responses. We measured synchrony between demonstrators' and observers' phasic electrodermal signals during learning, which directly reflects autonomic nervous system activity. Prior interpersonal synchrony predicted the strength of the observer's later skin conductance responses to threat predicting stimuli, in the absence of the demonstrator. Dynamic coupling between an observer's and a demonstrator's autonomic nervous system activity may reflect experience sharing processes facilitating the formation of observational threat associations.