Malnutrition is a serious problem in older adults, particularly for those at risk of hospital readmission. The essential step in managing malnutrition is early identification using a valid nutrition screening tool. The purpose of this study was to validate the Malnutrition Screening Tool (MST) in older adults at high risk of hospital readmission. Two RNs administered the MST to identify malnutrition risk and compared it with the comprehensive Subjective Global Assessment (SGA) to assess nutritional status for patients 65 and older who had at least one risk factor for hospital readmission. The MST demonstrates substantial sensitivity, specificity, and agreement with the SGA. These findings indicate that nursing staff can use the MST as a valid tool for routine screening and rescreening to identify patients at risk of malnutrition. Use of the MST may prevent hospital-acquired malnutrition in acute hospitalized older adults at high risk of readmission.