Detecting cooperative partners in situations that have financial stakes is crucial to successful social exchange. We tested whether humans are sensitive to subtle facial dynamics of counterparts when deciding whether to trust and cooperate. Participants played a 2-person trust game prior to which the facial dynamics of the other player were manipulated using brief (< 6 seconds) but highly realistic facial animations. Results showed that facial dynamics significantly influenced participants’ (a) choice of whom to play the game with and (b) decisions to cooperate. We also found that inferences about the other player’s trustworthiness mediated these effects of facial dynamics on cooperative behavior.