Impostorism is characterized by feelings of self-doubt regarding one's competence, ability, and deservingness of past achievements, despite evidence of competence. Impostorism has been shown to be associated with numerous adverse academic and psychosocial outcomes. However, there is limited research investigating these relationships within the general undergraduate student population. Furthermore, there is a lack of research examining the link between impostorism and academic help-seeking. The current study aims to fill the gaps by investigating the roles of impostorism and academic help-seeking attitudes and behaviors in undergraduate students' academic sense of belonging and college completion intention. A total of 2,808 diverse groups of undergraduate students from two universities in the U.S. were recruited in the study. Regression analyses demonstrated that higher levels of impostorism were linked to negative attitudes toward academic help-seeking, increased academic help-seeking avoidance behaviors, decreased academic sense of belonging, and lower intention to complete undergraduate studies. Additionally, mediation analyses revealed academic help-seeking attitudes and behaviors as the mediating factors through which impostorism affects academic sense of belonging and college completion intention. Implications and future directions are discussed.