Objective: Appropriate weight gain during pregnancy is important for optimal perinatal outcomes. In 2009, Institute of Medicine (IOM) provided guidelines for gestational weight gain (GWG). However, there has been little information about that the guidelines are valid for twin pregnancies in Korea. The purpose of this study is to investigate the effect of GWG on neonatal outcomes by IOM guidelines for twin pregnancies. Methods: A retrospective cohort study included women with twin pregnancy who delivered at Seoul National University Bundang Hospital. The subjects were divided in to three groups based on weight gain per gestational week by the 2009 IOM guidelines: women who gained below, within, and above the guidelines. We compared the neonatal outcomes of each group and calculated the adjusted odds ratios by multiple logistic regression analysis. Results: A total of 1490 twin pregnancies were included, of whom 771 (51.7%), 568 (38.1%) and 151 (10.1%) were gained below, within, and above the guidelines, respectively. The rate of SGA was significantly higher in neonates of women who gained below the guidelines compared to those who gained within (14.5% vs. 10.2%, p<0.01). The rate of LGA showed significant differences according to the GWG categorizations, which was the highest in those who gained above the guidelines (3.1% vs. 7.0% vs. 11.9%, p<0.001). After adjusting for confounders, women who gained below the guidelines had higher odds of preterm birth and SGA (aOR 2.553 and 1.375, p<0.05) and lower odds of LGA (aOR 0.460, p<0.001) and women who gained above the guidelines had higher odds of LGA (aOR 1.662, p<0.05) as compared with women who gained within. Conclusion: The 2009 IOM recommendations of GWG can be applied to Korean women with twin pregnancy for optimal neonatal outcomes. However, over a half of women were categorized as inadequate weight gain according to the guidelines. Further studies for the Korean national references of GWG in twin pregnancies should be constructed.