Background: Anxiety and depression are common during pregnancy and associated with adverse outcomes for the mother and infant if left untreated. Despite the need to improve treatment accessibility and uptake in this population, no studies have investigated internet-delivered cognitive behavioural therapy (iCBT) for antenatal anxiety and depression. In a randomised controlled trial, we examined the efficacy and acceptability of a brief, unguided iCBT intervention – the MUMentum Pregnancy program – in pregnant women with anxiety and/or depression. Methods: Participants meeting clinical threshold on validated self-report measures of generalised anxiety and/or depression were recruited online and randomised to iCBT (n=43) or a treatment as usual (TAU) control (n=44). Outcomes were assessed at baseline, post-treatment and four-week follow-up; and included anxiety, depression, psychological distress, antenatal bonding, quality of life, and treatment acceptability. Results: Of the 36 women who started iCBT, 26 completed all three lessons of treatment (76% adherence rate). iCBT produced moderate to large effect size reductions for anxiety on the GAD-7 (Hedges’ g=0.76) and psychological distress on the Kessler-10 (g=0.88) that were superior to TAU. Only small nonsignificant differences were found for depression outcomes (g=<0.35). Participants reported that iCBT was an acceptable treatment for antenatal anxiety and/or depression. Limitations: Lack of an active control condition and long-term postpartum follow-up. Conclusions: This is the first study to evaluate brief unguided iCBT for antenatal anxiety and depression. While our findings are promising, particularly for anxiety reduction, additional RCTs are required to establish treatment efficacy.