The objectives of this study were to evaluate the efficacy of a health coaching (HC) intervention designed to prevent excessive gestational weight gain (GWG), and promote positive psychosocial and motivational outcomes in comparison with an Education Alone (EA) group. Randomized-controlled trial. Two hundred and sixty-one women who were <18 weeks pregnant consented to take part. Those allocated to the HC group received a tailored HC intervention delivered by a Health Coach, whilst those in the EA group attended two education sessions. Women completed measures, including motivation, psychosocial variables, sleep quality, and knowledge, beliefs and expectations concerning GWG, at 15 weeks of gestation (Time 1) and 33 weeks of gestation (Time 2). Post-birth data were also collected at 2 months post-partum (Time 3). There was no intervention effect in relation to weight gained during pregnancy, rate of excessive GWG or birth outcomes. The only differences between HC and EA women were higher readiness (b = 0.29, 95% CIs = 0.03-0.55, p < .05) and the importance to achieve a healthy GWG (b = 0.27, 95% CIs = 0.02-0.52, p < .05), improved sleep quality (b = -0.22, 95% CIs = -0.44 to -0.03, p < .05), and increased knowledge for an appropriate amount of GWG that would be best for their babys health (b = -1.75, 95% CI = -3.26 to -0.24, p < .05) reported by the HC at Time 2. Whilst the HC intervention was not successful in preventing excessive GWG, several implications for the design of future GWG interventions were identified, including the burden of the intervention commitment and the use of weight monitoring. What is already known on the subject? Designing interventions to address gestational weight gain (GWG) continues to be a challenge. To date, health behaviour change factors have not been the focus of GWG interventions. What does this study add? Our health coaching (HC) intervention did not reduce GWG more so than education alone (EA). There was an intervention effect on readiness and importance to achieve healthy GWG. Yet there were no group differences regarding confidence to achieve healthy GWG post-intervention.