This study examined the presence of pluralistic ignorance about diversity beliefs and its relationship with relational conflict, using group-level survey data. We hypothesized that employees would misperceive their colleagues’ diversity beliefs and estimate them as less positive than their personal beliefs, and that this would cause conflict in the workplace. We surveyed 514 employees at a manufacturing company in Japan regarding 1) their personal diversity beliefs, 2) their perception of colleagues’ diversity beliefs, and 3) their relational and task conflict in the workplace. As hypothesized, the employees’ perceptions of their colleagues’ diversity beliefs were more negative than both their personal beliefs (misalignment) and the average beliefs of colleagues who worked alongside them (misperception). The misalignment and misperception scores had intraclass correlations (rwg), implying that employees in the same workplace misaligned and misperceived their colleagues’ beliefs to the same degree. We also found that the more employees perceived the misalignment, the more they felt relational conflicts with their colleagues. The results demonstrate pluralistic ignorance about diversity beliefs in an organization and its negative consequences in the form of higher relational conflict.