The present research investigated how different dimensions of narcissism (i.e., assertive, antagonistic, and vulnerable) and content-specific forms of assertive narcissism (i.e., intellectual ability, physical attractiveness, social dominance) are related to overclaiming bias (i.e., the tendency to illegitimately claim knowledge). In the data from a large-scale online study (N = 1,658), the associations between overclaiming bias and any kind of narcissism were smaller than in many previous studies. Furthermore, assertive narcissism was more positively related to overclaiming bias than antagonistic and vulnerable narcissism were. Intellectual-ability-specific and social-dominance-specific assertive narcissism were more positively related to overclaiming bias than physical-attractiveness-specific assertive narcissism was. Finally, multiple regression analyses suggested that the narcissism-overclaiming link is most robust for social-dominance-specific assertive narcissism.