BackgroundHyperemesis gravidarum (HG) affects about 2% of pregnancies and is at the severe end of the spectrum of nausea and vomiting of pregnancy. HG causes severe maternal distress and results in adverse pregnancy outcomes long after the condition may have dissipated. Although dietary advice is a common tool in management, trial evidence to base the advice on is lacking.MethodsA randomized trial was conducted in a university hospital from May 2019 to December 2020. 128 women at their discharge following hospitalization for HG were randomized: 64 to watermelon and 64 to control arm. Women were randomized to consume watermelon and to heed the advice leaflet or to heed the dietary advice leaflet alone. A personal weighing scale and a weighing protocol were provided to all participants to take home. Primary outcomes were bodyweight change at the end of week 1 and week 2 compared to hospital discharge.ResultsWeight change (kg) at end of week 1, median[interquartile range] -0.05[-0.775 to + 0.50] vs. -0.5[-1.4 to + 0.1]P= 0.014 and to the end of week 2, + 0.25[-0.65 to + 0.975] vs. -0.5[-1.3 to + 0.2]P= 0.001 for watermelon and control arms respectively. After two weeks, HG symptoms assessed by PUQE-24 (Pregnancy-Unique Quantification of Emesis and Nausea over 24 h), appetite assessed by SNAQ (Simplified Nutritional Appetite Questionnaire), wellbeing and satisfaction with allocated intervention NRS (0–10 numerical rating scale) scores, and recommendation of allocated intervention to a friend rate were all significantly better in the watermelon arm. However, rehospitalization for HG and antiemetic usage were not significantly different.ConclusionAdding watermelon to the diet after hospital discharge for HG improves bodyweight, HG symptoms, appetite, wellbeing and satisfaction.Trial registrationThis study was registered with the center’s Medical Ethics Committee (on 21/05/2019; reference number 2019327–7262) and the ISRCTN on 24/05/2019 with trial identification number:ISRCTN96125404. First participant was recruited on 31/05/ 2019.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12884-023-05771-7.