People who perceive they lack power inhibit their needs and goals, sometimes aggress to restore power, and experience poorer well-being. However, people may underestimate how much power they have to meet their needs. Guided by error management principles, we tested whether people systematically underestimate their power in relationships. Across four samples of friendships, same-gender couples, and woman-man couples (N = 1,304 dyads), we used Truth and Bias models to assess discrepancies between people's own perceived power and the power they had as reported by their friends/partners. We found robust evidence that people underestimated their power. Moreover, higher self-protection motives (e.g., attachment anxiety) and specific power motives (e.g., desire for power) predicted greater underestimation bias whereas higher pro-relationship motives (commitment) predicted lower underestimation bias. These results illustrate that self-protection, pro-relationship, and power motives bias perceptions of power, advancing our understanding of why and how these predictors shape power-related behaviors and relationship outcomes.