Background <p>Peritoneal lavage cytology (PLC) is used to detect peritoneal micrometastases in gastrointestinal malignancies, but its prognostic value in colorectal cancer (CRC) remains controversial, particularly in stage II–III. We aimed to evaluate the prognostic significance of PLC in predicting postoperative outcomes and peritoneal metastasis risk among patients with stage II–III CRC, thereby facilitating individualized treatment strategies.</p> Patients and Methods <p>A retrospective analysis was performed on 375 patients with stage II–III CRC who underwent curative resection with standardized intraoperative PLC between January 2017 and August 2018. Survival outcomes, including 5-year disease-free survival (DFS), overall survival (OS), and cancer-specific survival (CSS), were assessed using Kaplan–Meier analysis and Cox regression models. A prognostic nomogram incorporating PLC status along with other clinicopathological factors was developed and validated using area under the curve (AUC), <i>C</i>-index, and calibration curves. A competing risk analysis was performed to further elucidate the association between PLC positivity and specific metastasis patterns.</p> Results <p>PLC positivity, observed in 9.9% of patients, predicted reduced 5-year DFS and CSS, particularly in stage II patients, but not in stage III patients. Multivariate analysis identified PLC positivity as an independent prognostic factor for DFS (HR = 2.35, 95% CI 1.11–4.95, <i>P</i> = 0.025), along with preoperative CA199, tumor location, lymphovascular invasion, and N stage. The prognostic model demonstrated excellent discrimination (<i>C</i>-index = 0.796) and calibration. In addition, PLC-positive patients exhibited a significantly higher risk of peritoneal metastasis.</p> Conclusions <p>PLC is a valuable prognostic tool for the early detection of micrometastatic risk and peritoneal dissemination in stage II–III CRC, particularly in stage II disease.</p>