This study aimed to verify the relationship between assignment of professional registered dietitians (RDs) and other healthcare professionals and body weight or functional outcome in underweight patients. This was a secondary analysis of the nation-wide survey data from Kaifukuki (convalescent) rehabilitation wards (KRWs). Data of patients aged ≥20 y with disabilities and body mass index (BMI) <18.5 kg/m² and who were discharged from 1,099 KRWs were analyzed. The primary outcome was BMI at discharge. Secondary outcomes were Functional Independence Measure (FIM) at discharge and returning to home. Patients were divided into two groups: those in KRWs with ≥1 or <1 dedicated RD per ward (KRW/RD+ and KRW/RD−, respectively). Of 5,843 eligible participants (female, 63%; median age, 82 y; hip/vertebral/knee fracture, 47%; stroke, 34%; disuse syndrome secondary to acute illness, 11%; others, 8%), 1,288 and 4,555 were from the KRW/RD+ and KRW/RD− groups, respectively. At discharge, KRW/RD+ patients had higher FIM (93 vs. 90) and BMI (17.1 vs. 17.0 kg/m²) than did KRW/RD− patients. Multivariable analysis showed that assignment of dedicated RDs ( B =0.213, 95% confidence interval [CI], 0.036-0.389), number of nurses ( B =0.023, 95% CI, 0.003-0.043), and daily rehabilitation dose were significantly associated with changes in body weight. Furthermore, these factors positively affected BMI at discharge. Number of nurses and rehabilitation dose correlated with FIM, but assignment of RDs did not correlate with FIM. In conclusion, assignment of RDs, nurses, and sufficient rehabilitation dose may contribute to BMI gain. Nurses and daily rehabilitation dose may positively affect functional recovery.