Background/Objectives: Assertiveness, defined as the positive affirmation of oneself, encompasses the ability to refuse requests, express anger, disagree or oppose others, show affection, and uphold personal beliefs without causing conflict. Deficits in assertive behavior are often linked to pathological changes in the basal ganglia and prefrontal dopaminergic systems, commonly observed in Parkinson's disease (PD), and are predictive of poor clinical outcomes. Psychological factors such as mood alterations and cognitive dysfunction may also impact assertiveness. This study investigated the psychological factors influencing assertiveness in individuals with PD. Methods: A cross-sectional study was conducted, involving 160 patients with PD attending a movement disorders outpatient clinic. The participants underwent assessment using a battery of standardized neuropsychological tests to evaluate cognitive function, assertiveness, mood, dysarthria, and quality of life (QoL). Results: All dimensions of assertiveness correlated with depression and anxiety. Individuals experiencing mood disturbances may struggle to express themselves assertively. Similarly, some dimensions of assertiveness correlated also with the QoL, indicating that, overall, well-being affects assertive behavior. Gender emerged as a significant influencer of assertiveness across all dimensions. Specifically, in subjects with PD, the male gender was associated with lower scores in assertiveness compared to women. No significant correlations were found between assertiveness and dysarthria. Conclusions: The findings highlight the importance of adopting a holistic approach to PD management, addressing not only motor symptoms but also psychological challenges which patients may encounter in their daily lives.