Existing trust research has disproportionately focused on what makes people more or less trusting, and has largely ignored the question of what makes people more or less trustworthy. In this investigation, we deepen our understanding of trustworthiness. Across six studies using economic games that measure trustworthy behavior and survey items that measure trustworthy intentions, we explore the personality traits that predict trustworthiness. We demonstrate that guilt-proneness predicts trustworthiness better than a variety of other personality measures, and we identify sense of interpersonal responsibility as the underlying mechanism by both measuring it and manipulating it directly. People who are high in guilt-proneness are more likely to be trustworthy than are individuals who are low in guilt-proneness, but they are not universally more generous. We demonstrate that people high in guilt-proneness are more likely to behave in interpersonally sensitive ways when they are more responsible for others’ outcomes. We also explore potential interventions to increase trustworthiness. Our findings fill a significant gap in the trust literature by building a foundation for investigating trustworthiness, by identifying a trait predictor of trustworthy intentions and behavior, and by providing practical advice for deciding in whom we should place our trust.