IntroductionCurrent pharmacological treatment options for hyperemesis gravidarum have been introduced based on scarce evidence and are often not sufficiently effective. Several case reports suggest that mirtazapine, an antidepressant, may be an effective treatment for hyperemesis gravidarum, but so far there are no controlled trials investigating the potential effect of mirtazapine on hyperemesis gravidarum. The antiemetic ondansetron is currently widely used to treat hyperemesis gravidarum despite sparse evidence of effect in pregnant women. This study aims to investigate the effect of mirtazapine on hyperemesis gravidarum while also providing data on the effect of ondansetron.Methods and analysisThis randomised double-blind placebo-controlled multicentre trial will be conducted in eight Danish hospitals. One hundred and eighty pregnant women referred to secondary care for hyperemesis gravidarum will be randomly allocated to 14-day treatment with either mirtazapine, ondansetron or placebo. Main inclusion criterion will be Pregnancy Unique Quantification of Emesis (PUQE-24) score ≥13 or PUQE-24 score ≥7 if accompanied by weight loss >5% of pre-pregnancy weight or hospitalisation. Participants are eligible regardless of whether other antiemetics, including ondansetron, have been tried. The coprimary outcomes are effects of mirtazapine and ondansetron, respectively, on PUQE-24 score tested hierarchically on day 2 and day 14. Secondary outcomes include, but are not limited to, differences between the three groups in number of daily vomiting episodes, dropout due to treatment failure, use of rescue medication, weight change and side effects.Ethics and disseminationThe trial has been approved by the Regional Committees on Health Research Ethics in the Capital Region of Denmark, the Danish Medicines Agency and the Danish Data Protection Agency. Results will be published in peer-reviewed journals and submitted to relevant conferences.Trial registration numberNCT03785691