Maternal overweight is a risk factor for child obesity. Breastfeeding may decrease this risk, but breastfeeding prevalence is low in overweight or obese mothers. We conducted a randomized trial in 90 overweight/obese pregnant women in Bogotá-Colombia during 2019, to evaluate the effects of a new exclusive breastfeeding (EBF) counselling intervention for overweight/obese mothers, based on Carl Rogers' client-centered theory. The Intervention included individualized breastfeeding counseling, empowerment sessions, and a set of problem-solving strategies based on Carl Rogers' client-centered theory, conducted during late pregnancy, first week postpartum, 1 and 3 months postpartum. Primary outcomes were EBF during the last 24 h prevalence at 4 months postpartum, infant growth, and maternal weight loss at 4 months postpartum; secondary outcomes were serum and breast milk prolactin concentration, breast milk energy and macronutrient content, estimated breast milk volume at 1 and 4 months and EBF prevalence at interim time-points. Mothers were randomised in late pregnancy to intervention (new breastfeeding counselling; IG) or control group (standard breastfeeding support; CG). The IG had significantly higher EBF prevalence at 4 months (82.8%) compared to the CG (30.6%) (Prevalence ratio or PR = 2.7; 95% CI = 1.6, 4.5). There were no intervention effects on infant growth velocity, maternal weight loss or secondary outcomes. The intervention, which could be implemented in primary care settings, was highly effective for increasing the prevalence of EBF in overweight/obese mothers at 4 months postpartum. The results should, however, be interpreted in the context of the small sample size, short follow-up period and loss to follow-up. Further evaluation of the intervention is required in a larger sample including longer-term infant follow-up. (UTN) U1111-1228-9913 20 February 2019; ISRCTN15922904, retrospectively registered.