Objective: To determine if both gestational diabetes mellitus (GDM) and maternal overweight/obesity are independently associated with delivery of large-for-gestational-age (LGA) babies in Taiwan. Materials and methods: Anthropometric parameters were measured and 75-g oral glucose-tolerance tests were administered to a cohort of 1428 pregnant women at 24–28 weeks gestation at nine hospitals in Taiwan. GDM was diagnosed based on the International Association of Diabetes and Pregnancy Study Groups criteria. Reported pre-pregnancy BMI and measured BMI during pregnancy were recorded at the late stage of the second trimester and the third trimester. Neonatal anthropometrics were measured at delivery. Primary outcome was LGA, defined in this study as having a birth weight ≥90th percentile for gestational age defined by WHO or a Chinese growth reference, taking into consideration the racial/ethnic and environmental differences in growth around the world. Multiple logistic regression was used to examine associations of GDM and maternal overweight/obesity with outcomes. Results: Based on WHO growth reference definition of LGA, subjects with pre-pregnancy BMI ≥24 and pregnancy BMI >28.4 were found to be 2.46 times (0.76–7.97) and 3.28 times (1.01–10.60), respectively, more likely to deliver LGA babies than subjects with normal pre-pregnancy and pregnancy BMIs. Compared to those without GDM, subjects with GDM were 7.55 (1.62–35.25) times more likely to deliver LGA babies. The odds ratios for delivering a baby with a birth weight ≥90th percentile were 11.40 (1.65–78.75) for those with GDM alone, 4.10 (1.07–15.65) for those with overweight/obesity alone and 15.75 (1.30–190.40) for those with both GDM and overweight/obesity, compared to those with no GDM and no overweightness. Women with both pre-pregnancy and pregnancy overweightness/obesity were 3.64 (1.07–12.34) times more likely to deliver LGA. The above results remained similar when analyzing data based on Chinese growth reference definition of LGA. Conclusion: Maternal overweightness/obesity and GDM are independently associated with LGA. Their combination had a greater impact than either one alone.