Most decisions are accompanied by a feeling of confidence, i.e., a subjective estimate of the probability of being correct. Although confidence accuracy is critical, notably in high-stakes domains such as medical or financial decision-making, little is known about how incentive motivation influences this metacognitive judgment. In this article, we hypothesized that motivation can, paradoxically, deteriorate confidence accuracy. We designed an original incentive-compatible perceptual task to investigate the effects of monetary incentives on human confidence judgments. In line with classical theories of motivated cognition, our results first reveal that monetary incentives improve some aspects of confidence judgments. However, over three experiments and in line with our hypothesis, but unpredicted by normative or classical motivated cognition theories, we further show that incentives also robustly bias confidence reports: the perspective of potential gains (respectively losses) bias confidence upward (respectively downward), with potential detrimental consequences on confidence accuracy. Connecting our findings with recent models of confidence formation, we demonstrate that these two effects of incentives have dissociable signatures on how confidence builds on decision evidence. Altogether, these findings enrich current cognitive and evolutionary models of confidence, and may provide new hints about its healthy or pathological miscalibration.