Excess gestational weight gain (GWG) among women with overweight or obesity synergistically increases their already elevated risk of having gestational diabetes, a caesarean delivery, a large for gestational age infant, and post-partum weight retention, and increases their child’s risk of obesity. We investigated whether a primarily telehealth lifestyle intervention reduced excess GWG among women with overweight or obesity. We did a randomised controlled trial in five antenatal clinics of Kaiser Permanente; Oakland, San Leandro, Walnut Creek, Fremont, and Santa Clara, CA, USA. Women at 8-15 weeks’ gestation with singletons, pre-pregnancy BMI 25·0-40·0 kg/m2, and aged 18 years or older were randomly assigned (1:1) to receive the telehealth lifestyle intervention or usual antenatal care. Randomisation was adaptively balanced for age, BMI, and race and ethnicity. Data collectors and investigators were masked to group assignments. The core lifestyle intervention consisted of two in-person and 11 telephone sessions on behavioural strategies to improve weight, diet, and physical activity, and stress management to help women meet a trial goal of gaining at the lower limit of the Institute of Medicine (IOM) guidelines range for total GWG: 7 kg for women with overweight and 5 kg for women with obesity. Usual antenatal care included an antenatal visit at 7-10 weeks’ gestation, an additional seven antenatal visits, on average, and periodic health education newsletters, including the IOM GWG guidelines and information on healthy eating and physical activity in pregnancy. The primary outcome was weekly rate of GWG expressed as excess GWG, per Institute of Medicine guidelines and mean assessed in the intention-to-treat population. The trial is registered at ClinicalTrials.gov, NCT02130232. Between March 24, 2014, and Sept 26, 2017, 5329 women were assessed for eligibility and 200 were randomly assigned to the lifestyle intervention group and 198 to the usual care group. Analyses included 199 women in the lifestyle intervention group (one lost to follow-up) and 195 in the usual care group (three lost to follow-up). 96 (48%) women in the lifestyle intervention group and 134 (69%) women in the usual care group exceeded Institute of Medicine guidelines for rate of GWG per week (relative risk 0·70, 95% CI 0·59 to 0·83). Compared with usual care, women in the lifestyle intervention had reduced weekly rate of GWG (mean 0·26 kg per week [SD 0·15] vs 0·32 kg per week [0·13]; mean between-group difference -0·07 kg per week, 95% CI -0·09 to -0·04). No between-group differences in perinatal complications were observed. Our evidence-based programme showed that health-care delivery systems could further adapt to meet the needs of their clinical settings to prevent excess GWG and improve healthy behaviours and markers of insulin resistance among women with overweight or obesity by using telehealth lifestyle interventions. US National Institutes of Health.