Background/Objectives: In the elderly population, thoracic kyphosis often progresses with age, leading to secondary postural adaptations including scapular protraction, internal rotation, and anterior tilt. These alterations can potentially compromise shoulder biomechanics, particularly in patients undergoing reverse shoulder arthroplasty (RSA). The purpose of this study was to evaluate the relationship between thoracic sagittal alignment, quantified by the Cobb angle, and scapular internal rotation (SIR) assessed on CT scans in patients scheduled for RSA. Methods: A retrospective study was conducted on 164 patients who underwent RSA between 2016 and 2024 at a single tertiary referral center. Sagittal thoracic kyphosis was assessed using the Cobb angle measured on preoperative chest radiographs. SIR and anterior scapular tilt were evaluated using preoperative CT scans. Patients were divided into three groups according to the Cobb angle: Group A (≤36°), Group B (>36–46°), and Group C (≥47°). Statistical analysis was performed using the Spearman correlation coefficient and Kruskal–Wallis test, with a significance threshold set at p < 0.05. Results: Analysis demonstrated a weak but statistically significant positive correlation between age and SIR, as well as between thoracic kyphosis (Cobb angle) and SIR. Patients in Group C (Cobb angle ≥ 47°) showed higher mean SIR values (43.7°) compared to Group A (40.3°), with statistical significance achieved (p = 0.047). These findings suggest that greater thoracic kyphosis is associated with increased scapular internal rotation. No significant correlation was identified between anterior scapular tilt and thoracic kyphosis. Conclusions: This study reveals a correlation between increased thoracic kyphosis and greater scapular internal rotation in patients undergoing RSA. These postural and biomechanical alterations may have critical implications for surgical planning and postoperative outcomes. Preoperative assessment of sagittal spinal alignment, particularly thoracic kyphosis, should be integrated into the planning process for RSA to optimize implant positioning and improve functional results.