Misinformation represents one of the greatest challenges for the functioning of societies in the information age. Drawing on a signal-detection framework, the current research investigated two distinct aspects of misinformation susceptibility: truth sensitivity, conceptualized as accurate discrimination between true and false information, and partisan bias, conceptualized as lower acceptance threshold for ideology-congruent information compared to ideology-incongruent information. Four preregistered experiments (n = 2,423) examined (a) truth sensitivity and partisan bias in veracity judgments and decisions to share information and (b) determinants and correlates of truth sensitivity and partisan bias in responses to misinformation. Although participants were able to distinguish between true and false information to a considerable extent, sharing decisions were largely unaffected by actual information veracity. A strong partisan bias emerged for both veracity judgments and sharing decisions, with partisan bias being unrelated to the overall degree of truth sensitivity. While truth sensitivity increased as a function of cognitive reflection during encoding, partisan bias increased as a function of subjective confidence. Truth sensitivity and partisan bias were both associated with misinformation susceptibility, but partisan bias was a stronger and more reliable predictor of misinformation susceptibility than truth sensitivity. Implications and open questions for future research are discussed.