Nomograms for predicting survival are well-established for colorectal adenocarcinoma but are less common for rectal mucinous adenocarcinoma (MAC). This study leverages Surveillance, Epidemiology, and End Results (SEER) database to develop a prognostic nomogram for rectal MAC, enhancing survival prediction and clinical decision-making. Data from the SEER (2004–2017) were analyzed, focusing on surgically treated rectal MAC patients. The study excluded cases with inadequate follow-up or incomplete data. Demographic and pathological variables, along with treatment modalities (radiotherapy and chemotherapy), were examined. A nomogram was developed to predict 1-, 3-, and 5-year cancer-specific survival (CSS), and its accuracy was validated using receiver operating characteristic (ROC) curves, calibration, and decision curve analysis (DCA). The study included 690 patients, segmented into training (483) and validation (207) cohorts. Significant factors affecting survival included age and cancer stage. The resulting nomogram, incorporating age and TNM stages, demonstrated a concordance index of 0.722 and was further validated through ROC analysis, calibration, and DCA, showing robust predictive performance and clinical utility. Age and TNM staging are critical predictors of CSS in rectal MAC patients post-surgery. The developed nomogram, utilizing these factors, offers improved prognostic accuracy and is a valuable tool for clinical practice, facilitating more informed treatment decisions.