Witnessing another person's suffering elicits vicarious brain activity in areas active when we ourselves are in pain. Whether this activity influences prosocial behavior remains debated. Here participants witnessed a confederate express pain via a reaction of the swatted hand or via a facial expression and could decide to reduce that pain by donating money. Participants donate more money on trials in which the confederate expressed more pain. EEG shows that activity of the SI hand region explains variance in donation; TMS shows that altering this activity interferes with the pain-donation coupling only when pain is expressed by the hand and HD-tDCS that altering SI activity also interferes with pain perception. These experiments show vicarious somatosensory activations contribute to prosocial decision-making and suggest they do so by helping transform observed reactions of affected body-parts into accurate perceptions of pain that are necessary for decision making.