The aim of this study was to determine the effects of pre-gestational body mass index on pregnancy outcomes of women with gestational diabetes in Japan. A multi-institutional retrospective study was performed. We examined pregnant women who met the former criteria for gestational diabetes in Japan, receiving dietary intervention with self-monitoring of blood glucose with or without insulin therapy. Women with gestational diabetes were divided into three groups according to pre-gestational body mass index: body mass index <25 (control group), 25 ≤ body mass index <30 (overweight group), body mass index ≥30 (obese group). Data from a total of 1,758 eligible women were collected from 40 institutions. Participants included 960 controls, 426 overweight women, and 372 obese women with gestational diabetes. Gestational weight gain was highest in the control and lowest in the obese group. The prevalences of chronic hypertension and pregnancy induced hypertension were higher in the overweight and obese groups than in the control group. Multiple logistic regression analysis revealed pre-gestational body mass index, gestational weight gain, chronic hypertension, and nulliparity to be associated with the onset of pregnancy induced hypertension, while the 75-g OGTT results were unrelated to pregnancy induced hypertension. The prevalence of large-for-gestational age was lower in infants born to obese women than in those born to overweight or control women. The present results suggest that medical interventions for obese women with gestational diabetes may contribute to reducing the prevalence of large-for-gestational age but would not achieve marked reductions in maternal complications.