Purpose <p>Prolonged sedentary behavior and slump sitting posture may impair neuromuscular function; however, acute effects on postural sway and potential differences between athletes and non-athletes remain unclear. Therefore, this study aims to examine the acute effects of prolonged slump sitting on postural sway in female athletes and non-athletes.</p> Methods <p>In this study, 24 females (12 athletes, 12 non-athletes; age 18–30 years) completed balance assessments using the Biodex Balance System, followed by a 30-minute standardized slump sitting protocol. Outcomes included static postural stability test (PST), Modified Clinical Test of Sensory Interaction on Balance (mCTSIB), and limits of stability (LOS). Group differences were analyzed using analysis of covariance (ANCOVA).</p> Results <p>Prolonged slump sitting significantly influenced selected postural sway outcomes. Athletes demonstrated greater post-intervention sway compared with non-athletes in the Overall Stability Index (<i>p</i> = 0.01) and the eyes-closed firm-surface mCTSIB condition (<i>p</i> = 0.02). A significant group difference was also observed in static left LOS performance (<i>p</i> = 0.01).</p> Conclusion <p>In a nutshell, the present study demonstrated that prolonged slump sitting may influence postural sway in female athletes and non-athletes, with specific differences emerging between groups under certain sensory and stability conditions. By highlighting the potential impact of slump sitting on postural stability, this research contributes to the fields of sports science, ergonomics, and rehabilitation, emphasizing the need for strategies that mitigate the negative effects of prolonged sedentary behavior.</p>