Background: The present study aimed to investigate the clinical correlates of treatment response to mood stabilizers in patients with bipolar disorder (BD), with a specific focus on trait-related characteristics such as impulsivity and affective temperaments. Methods: In- and outpatients diagnosed with BD were recruited at the Section of Psychiatry of the General Hospital/University of Perugia. Socio-demographic, clinical, and current psychopathological characteristics were collected. The treatment response was retrospectively assessed using the Alda Scale. Trait characteristics were evaluated through the Barratt Impulsiveness Scale (BIS-11) and the Brief Temperament Evaluation of Memphis, Pisa, and San Diego—Münster version (briefTEMPS-M). Bivariate analyses and a general linear model were employed to analyze the correlates of treatment response to mood stabilizers. Results: Among the investigated variables, trait impulsivity showed a significant negative association with treatment response. A similar effect was observed for depressive temperament, while other affective temperaments were not significantly associated with treatment outcomes. Patients with good treatment responses exhibited higher illness duration and lower severity of BD, higher prevalence of comorbid anxiety disorders, lower diurnal variation in depressive symptoms, and lower functional impairment in autonomy and occupational domains. The main limitations of this study were represented by the small sample size, the retrospective assessment of treatment response, and the inclusion of patients from a single center. Conclusions: The present findings suggest that impulsivity and depressive temperament should be investigated as potential predictors of poor response to mood stabilizers in BD. These trait dimensions, together with other clinical markers, may serve as useful targets for patient stratification and the development of personalized treatment strategies.