Background: SLN mapping has emerged as a standard of care in endometrial cancer due to its high sensitivity and significant reduction in morbidity. Although lymphovascular space invasion (LVSI) is a known risk factor for lymph node metastasis and recurrence, evidence on the reliability of SLN mapping in LVSI-positive patients is scarce. The aim of this study was to determine the impact of LVSI on the diagnostic performance of SLN mapping. Methods: This retrospective cohort study included patients with endometrial cancer who underwent primary surgical treatment at the Bern University Hospital, Switzerland, between 2012 and 2022. Results: LVSI was present in 22% of patients and was significantly associated with lymph node metastasis (p < 0.001) and recurrence (p < 0.001). In node-negative patients with only SLN mapping performed, LVSI was an independent predictor of recurrence during multivariable Cox regression analysis (p = 0.036). The negative predictive value of SLN mapping was 91.5% and was significantly lower in tumors with LVSI (75.0%) compared to LVSI-negative tumors (95.6%, p = 0.004). Conclusion: The presence of LVSI was significantly associated with worse oncological outcomes. LVSI was an independent predictor of recurrence in node-negative patients with only SLN mapping performed. Furthermore, the negative predictive value of SLN mapping was significantly lower in LVSI-positive tumors.