QUESTION:In women with gestational diabetes mellitus, does the addition of a consumer co-created infographic to usual education about gestational diabetes mellitus improve knowledge about physical activity and self-efficacy to exercise? DESIGN:A randomised trial with concealed allocation, a blinded assessor and intention-to-treat analysis. PARTICIPANTS:Sixty-nine women diagnosed with gestational diabetes mellitus. INTERVENTION:In addition to gestational diabetes education, the experimental group received a paper copy of a consumer co-created infographic about physical activity during a gestational diabetes pregnancy. The control group received gestational diabetes education alone. OUTCOME MEASURES:Participants completed outcome measures at baseline and again 1 week later. Knowledge of physical activity in a gestational diabetes mellitus pregnancy was assessed using a 19-item questionnaire modified to reflect current physical activity guidelines, with a total score from 0% (worst) to 100% (best). Self-efficacy was measured using the nine-item Self-Efficacy for Exercise Scale, with a total score from 0 (not confident) to 10 (very confident). RESULTS:Provision of the infographic led to a clinically important between-group difference in knowledge (MD 12%, 95% CI 10 to 15) and self-efficacy (MD 2.5 units, 95% CI 1.9 to 3.0). CONCLUSION:In women with gestational diabetes mellitus, short-term knowledge about physical activity and self-efficacy to exercise were improved when usual education was supplemented with a consumer co-created infographic that provided specific and relevant information about physical activity during a gestational diabetes mellitus pregnancy. TRIAL REGISTRATION:ACTRN12619001207101.