["Japanese Psychological Research, EarlyView. ", "\nAbstract\nDespite a long‐standing consensus on the positive impact of conscientiousness on life outcomes, its association with obsessive–compulsive (OC) symptoms remains controversial. This ambiguity complicates evidence‐based approaches to the prevention and intervention of OC symptoms. While conscientiousness is generally considered protective, excessively high levels may pose psychological risks, suggesting a potential curvilinear relationship with OC symptoms. This study examined the curvilinear relationships between conscientiousness and OC symptoms, with additional consideration of neuroticism and perfectionism. Data from 781 adults (388 males; 22.88 ± 3.19 years) were analyzed. Following data standardization and preliminary correlation analysis, we employed linear and quadratic regressions, two‐line tests, and polynomial regression analyses. Results revealed both linear and curvilinear relationships between conscientiousness and OC symptoms. Notably, the orderliness facet of conscientiousness showed robust positive associations with washing, neutralizing, and ordering symptoms. Furthermore, findings regarding neuroticism suggest that moderate indulgence and reasonable levels of emotional expression may be adaptive. These findings extend curvilinear research on conscientiousness and outcomes beyond the organizational domain, highlighting the importance of balance in conscientiousness and neuroticism for mental health.\n"]