Background: Malnutrition accounts for about 35% of under-five mortality, of which 4.4% is specifically because of severe wasting. Prevalence of vitamin D deficiency in malnourished children ranges from 31% to 61%. Addition of high dose vitamin D supplementation in children with severe acute malnutrition would be effective in elevating serum vitamin D concentration and thus improve weight gain. The association of vitamin D and malnutrition is well recognized but data are scarce. Objectives: To see the effect of vitamin D in the treatment of severe acute malnourished children. Methods: This prospective randomized controlled trial study was conducted among children 6 months to 5 years old with a diagnosis of SAM in the pediatric ward of Dhaka Shishu (Children) Hospital in Bangladesh from July 2018 to January 2020.Sample was collected through simple random sampling then randomized into two groups by lottery method. Finally, 50 patients were included in the study, among them 25 patients received 6 lac IU vitamin D orally in day 1 in addition to the standard treatment of SAM and the other 25 patients received only the standard treatment of SAM. Demographic data and other related information regarding clinical feature, laboratory feature, outcome variables were recorded and anthropometric measurement and Z-scoring was done to assess malnutrition. Collected data was checked manually and analyzed by computer-based program SPSS for Windows (version 20.0). Outcomes were: Length of hospital stay and average weight gain between the two groups. Comparison of parameters among the group were done by unpaired t-test and chi-square test. Results: There were no statistically significant difference in length of hospital stay (16.24 ±3.29 versus 15.34±2.64) days, p=0.291 but there was statistically significant difference in average weight gain (9.08±2.99 versus 7.52±2.06) gm/kg/day, p=0.037 and wasting (weight for height z score) improved significantly on follow ups (p=0.241 on first, p=0.039 on 2nd, p=0.009 on 3rd and p=<0.001 on 4th follow up). Conclusions: Vitamin D supplementation causes significant weight gain in children with severe acute malnutrition.