Introduction: The Dark Triad (DT), including narcissism, Machiavellianism, and psychopathy, represents the dark side of human nature and has been related to psychopathological symptoms (e.g., depression, anxiety, and stress). However, little is known about how the two constructs are related longitudinally. To fill this gap and to clarify the directionality between them, we conducted a longitudinal study. Methods: We measured DT traits and psychopathological symptoms in a large sample of university students (N-T1 = 1815) annually for 3 years. We implemented random intercept cross-lagged panel models in analysis. Results: Narcissism and psychopathological symptoms showed a reciprocal relationship at the within-person level: greater narcissism preceded a decline in psychopathological symptoms, while more severe symptoms preceded a decrease in narcissism. Within the same individual, increases in the DT, particularly psychopathy and Machiavellianism, were linked to concurrent escalations in the symptoms. Additionally, all DT traits were positively correlated with psychopathological symptoms as stable differences between individuals. Conclusions: This study constitutes an important step in clarifying the directionality between the DT and psychopathological symptoms, and advances our understanding of the interplay between these two constructs at both the between-person and within-person levels.