Background/Objectives: Malnutrition significantly hinders the recovery of patients undergoing convalescent rehabilitation. Proper nutritional management can improve rehabilitation outcomes. This study aimed to establish a novel nutritional screening method (J-Method) specifically for convalescent rehabilitation patients and compare it to the widely used Mini Nutritional Assessment Short Form (MNA-SF). Methods: We developed the J-Method for convalescent rehabilitation settings and compared its results with that of the MNA-SF. The J-Method comprised six items derived from various nutritional screening methods and obtained solely from medical records, without patient interviews. Data were collected from 148 patients aged > 65 years with cerebrovascular diseases admitted to a convalescent rehabilitation ward (CRW). Nutritional status was evaluated using the J-Method and MNA-SF, and results were compared. Results: The J-Method more precisely identified patients as malnourished than the MNA-SF. The results of nutritional screening using the J-Method identified patients in need of nutritional management intervention and suggested that, to improve the rehabilitation effect, nutritional management should be initiated in an acute hospital before admission to a CRW. Conclusions: The J-Method may be more effective than the MNA-SF for nutritional screening in convalescent rehabilitation settings as it provides a more accurate assessment of malnutrition without requiring patient interviews.