Interest in consensually non-monogamous relationships (CNM) has been increasing in the general population in recent years. However, given the cultural dominance of monogamy and the normative expectations often imposed through socialization (i.e., mononormativity), people in CNM relationships may experience negativity, which can become internalized and harm their individual and relationship health. The present study investigates if internalized mononormativity is associated with more self- and partner-directed stigma and if internalized CNM negativity is an underlying mechanism for these associations. Results showed that participants who endorsed more mononormativity beliefs also reported more internalized CNM negativity. In turn, participants who experienced more internalized negativity also reported more self-directed CNM stigma, attributed more negative (vs. positive) emotions to themselves, and treated their partners as more immature, unrefined, exploitable, and emotionless. These results show that mononormativity and internalized negativity are significant contributors to individual and relational functioning.