Background: Endometriosis is a common gynecological disorder. Non-invasive imaging markers that predict disease severity and bowel involvement are clinically valuable. The purpose of the current study was to evaluate the diagnostic performance of the kissing ovaries (KO) sign on transvaginal ultrasound for predicting bowel endometriosis and advanced disease. Methods: This retrospective diagnostic accuracy study included 252 women with surgically and histopathologically confirmed endometriosis who underwent laparoscopic surgery at Avicenna Fertility Center between 2019 and 2021. Preoperative transvaginal ultrasound findings were reviewed, and the presence of the KO sign was assessed. Associations with bowel endometriosis and disease severity were analyzed. Diagnostic performance metrics, chi-square and independent t-tests, ORs with 95% CIs, and multivariable logistic regression were used to evaluate predictors of bowel endometriosis. The p<0.05 was considered statistically significant. Results: The KO sign was significantly associated with advanced-stage disease (stage IV: 88.1% vs. 22.2%) and bowel involvement (61.1% vs. 12.7%; OR=10.3, 95%CI: 5.47–19.4). It also showed significant associations with pouch of Douglas obliteration, uterosacral ligament involvement, and fallopian tube involvement. No significant association was observed for bladder or appendiceal involvement. The KO sign demonstrated a sensitivity of 82.8%, specificity of 69.2%, and accuracy of 74.2% for predicting bowel endometriosis. In multivariable analysis, it remained an independent predictor of bowel involvement (adjusted OR=8.45, 95%CI: 4.62–15.48). The area under the ROC curve was 0.76. Conclusion: The KO sign on transvaginal ultrasound is a useful non-invasive imaging marker for predicting bowel endometriosis and advanced-stage disease, with good diagnostic performance and potential value in preoperative surgical planning.