Aim Excessive gestational weight gain increases risk of gestational diabetes mellitus (GDM) but it remains unclear whether weight control after GDM diagnosis improves outcomes. We assessed whether: (1) total gestational weight gain during pregnancy (0–36 weeks); (2) early gestational weight gain (0–28 weeks, before GDM diagnosis); or (3) late gestational weight gain (28–36 weeks, after diagnosis) are associated with maternal–fetal outcomes. Methods Some 546 women with GDM who delivered viable singleton infants at a single UK obstetric centre (October 2014 to March 2017) were included in this retrospective observational study. Results Higher total gestational weight gain was associated with Caesarean section n = 376; odds ratio (OR) 1.05; confidence intervals (CI) 1.02–1.08, P < 0.001 and large for gestational age (OR 1.08; CI 1.03–1.12, P < 0.001). Higher late gestational weight gain (28–36 weeks; n = 144) was associated with large for gestational age (OR 1.17; CI 1.01–1.37, P < 0.05), instrumental deliveries (OR 1.26; CI 1.03–1.55, P < 0.01), higher total daily insulin doses (36 weeks; beta coefficient 4.37; CI 1.92–6.82, P < 0.001), and higher post‐partum 2‐h oral glucose tolerance test concentrations (beta coefficient 0.12; CI 0.01–0.22, P < 0.05). Women who avoided substantial weight gain after GDM diagnosis had 0.7 mmol/l lower postnatal 2‐h glucose and needed half the amount of insulin/day at 36 weeks compared with women with substantial weight gain after diagnosis. There were no significant associations between early gestational weight gain (0–28 weeks) and pregnancy outcomes. Conclusions These findings suggest that controlling gestational weight gain should be a priority following GDM diagnosis to optimize pregnancy outcomes and improve maternal postnatal glucose homeostasis. The period after diagnosis of GDM (often 28 weeks gestation) is not too late to offer lifestyle advice or intervention to improve weight management and pregnancy outcomes. What's new? Gestational diabetes is associated with pre‐pregnancy obesity and excessive gestational weight gain. However, there is no clear evidence to guide weight management after diagnosis of gestational diabetes (at 28 weeks). In this study, excessive gestational weight gain was strongly associated with fetal growth and difficult deliveries in women with gestational diabetes. Women who gained weight after diagnosis needed higher insulin doses and had higher postnatal glucose concentrations. Keeping weight stable after diagnosis of gestational diabetes should be a priority to improve outcomes for both mother and child; 28 weeks is not too late to offer advice or intervention. A clinical trial to guide weight management in women with gestational diabetes would be extremely useful to confirm these findings and provide clear targets for weight gain after diagnosis.