We retrospectively analysed large-scale, nationwide data from the Kaifukuki (convalescent) Rehabilitation Ward (KRW) survey of 2015. Patients were classified into two groups based on changes in body mass index (BMI) during their KRW stay: increased BMI and non-increased BMI. The primary outcome was motor functional independence measure (FIM) score at discharge, and the secondary outcomes were motor FIM gain and full oral intake at discharge. We analysed 4605 patients (64% women; mean age, 79.3 years). Of these patients, 1128 and 3477 were classified into the increased and non-increased groups, respectively. Multivariate analysis showed that BMI increases were independently associated with motor FIM scores at discharge (partial regression coefficient = 1.165; 95% confidence interval, 0.671-1.659) and motor FIM gains, although BMI increase was not associated with full oral intake. Thus, increasing body weight might lead to activities of daily life improvement in underweight patients undergoing post-acute rehabilitation.