INTRODUCTION:The promotion of a healthy diet, physical activity and measurement of blood glucose levels are essential components in the care for women with gestational diabetes mellitus (GDM). Smartphones offer a new way to promote health behaviour. The main aim is to investigate if the use of the Pregnant+ app, in addition to standard care, results in better blood glucose levels compared with current standard care only, for women with GDM. METHODS AND ANALYSIS:This randomised controlled trial will include 230 pregnant women with GDM followed up at 5 outpatient departments (OPD) in the greater Oslo Region. Women with a 2-hour oral glucose tolerance test (OGTT) ≥9 mmol/L, who own a smartphone, understand Norwegian, Urdu or Somali and are <33 weeks pregnant, are invited. The intervention group receives the Pregnant+ app and standard care. The control group receives standard care only. Block randomisation is performed electronically. Data are collected using self-reported questionnaires and hospital records. Data will be analysed according to the intention-to-treat principle. Groups will be compared using linear regression for the main outcome and χ2 test for categorical data and Student's t-test or Mann-Whitney-Wilcoxon test for skewed distribution. The main outcome is the glucose level measured at the 2-hour OGTT 3 months postpartum. Secondary outcomes are a change in health behaviour and knowledge about GDM, quality of life, birth weight, mode of delivery and complications for mother and child. ETHICS AND DISSEMINATION:The study is exempt from regional ethics review due to its nature of quality improvement in patient care. Our study has been approved by the Norwegian Social Science Data Services and the patient privacy protections boards governing over the recruitment sites. Findings will be presented in peer-reviewed journals and at conferences. TRIAL REGISTRATION NUMBER:NCT02588729, Post-results.