Background <p>Psychological readiness is critical for successful return to sport (RTS) after shoulder instability, yet no validated Persian-language tool exists for this purpose. This study aimed to translate, adapt, and evaluate the psychometric properties of the Persian Shoulder Instability–Return to Sport Index (SI-RSI-Pr) in Iranian athletes.</p> Methods <p>A cross-sectional study was conducted with 93 athletes (mean age = 27.8 ± 5.0 years) with unilateral shoulder instability. The SI-RSI-Pr was translated using forward–backward procedures. Structural validity was assessed via exploratory factor analysis (EFA). Construct validity was evaluated through correlations with the Western Ontario Shoulder Instability Index (WOSI), Tampa Scale for Kinesiophobia (TSK-17), and Walch-Duplay Score (WDS). Known-groups validity compared athletes who returned to sport versus those who did not. Reliability was tested via internal consistency (Cronbach’s α) and test–retest reliability (ICC) over a 7–10-day interval.</p> Results <p>EFA confirmed a three-factor structure (Confidence in RTS, Fear and Anxiety about RTS, Psychological Burden of RTS), explaining 78% of variance with strong factor loadings (0.744–0.955) and good model fit (RMSEA = 0.070, TLI = 0.966). The SI-RSI-Pr showed high internal consistency (α = 0.89–0.94) and test–retest reliability (ICC = 0.90–0.95). Construct validity was supported by correlations with WDS (<i>r</i> = 0.74), TSK-17 (<i>r</i> = -0.58), and WOSI (<i>r</i> = -0.57). Known-groups validity was confirmed, distinguishing returners from non-returners (AUC = 0.765, cut-off = 61.67, specificity = 92.86%). The scale exhibited strong feasibility and minimal floor/ceiling effects.</p> Conclusions <p>The SI-RSI-Pr, rigorously adapted for Persian-speaking athletes, is a reliable and valid tool for assessing psychological readiness to RTS after shoulder instability, supporting return-to-sport decisions in Persian-speaking athletes.</p>