Background <p>The Supine-to-Stand Test (SST) evaluates trunk control, muscle strength, balance, and movement coordination.</p> Aims <p>This study investigated the reliability and validity of the SST in patients with Parkinson’s disease (pwPD).</p> Methods <p>Forty-one pwPD participated in this cross-sectional study. Ankle plantar and dorsiflexor strength was measured with a digital dynamometer. Manual dexterity, balance, and functional mobility were assessed using the 9-Hole Peg Test (9HPT), Berg Balance Scale (BBS), and Timed Up and Go Test (TUG). To examine the convergent validity of the Supine-to-Stand Test (SST). Receiver operating characteristic (ROC) curve analysis was performed, and the optimal cut-off value was determined based on the best combination of sensitivity and specificity. Fear of falling was evaluated with the Activity-specific Balance Confidence (ABC) scale. Motor severity was assessed via the Unified Parkinson’s Disease Rating Scale Part III (UPDRS-III), and quality of life with the Parkinson’s Disease Quality of Life Scale (PDQ-39). Test-retest reliability was determined using the intraclass correlation coefficient (ICC).</p> Results <p>SST test-retest reliability was excellent (ICC = 0.976, 95% CI: 0.955–0.976, <i>p</i> &lt; 0.001). Ankle dorsiflexor strength was moderately positively correlated with SST time bilaterally (dominant <i>r</i> = 0.381, <i>p</i> = 0.014; non-dominant <i>r</i> = 0.341, <i>p</i> = 0.029). Age, disease duration, plantar strength, hand functions, rigidity, and quality of life showed no significant relation with SST (<i>p</i> &gt; 0.05). A moderate negative correlation existed between BBS and SST (<i>r</i>=-0.341, <i>p</i> = 0.029), while TUG and SST (<i>r</i> = 0.351, <i>p</i> = 0.024) and ABC and SST (<i>r</i> = 0.384, <i>p</i> = 0.013) were moderately positively associated.</p> Conclusion <p>SST is a reliable and valid tool for assessing functional performance in pwPD.</p>