Rationale: Theoretically, most strokes could be prevented through the management of modifiable risk factors. The Stroke Riskometer™ mobile phone application (hereon “The App”) uses an individual’s data to provide personalized information and advice to reduce their risk of stroke. Aims: To determine the effect of The App on a combined cardiovascular risk score (Life’s Simple 7^®, LS7) of modifiable risk factors at 6 months post-randomization. Methods and design: PERKS-International is a Phase III, multicentre, prospective, pragmatic, open-label, single-blinded endpoint, two-arm randomized controlled trial (RCT). Inclusion criteria are as follows: age ⩾ 35 and ⩽75 years; ⩾2 LS7 risk factors; smartphone ownership; no history of stroke/myocardial infarction/cognitive impairment/terminal illness. The intervention group (IG) will be provided with The App, and the usual care group (UCG) is provided with generic online information about risk factors, but not be informed about The App. Face-to-face assessments will be conducted at baseline and 6 months, and online at 3 and 12 months. The RCT includes a process and economic evaluation. Study outcomes and sample size: The primary outcome is a difference in the mean change in LS7 (seven individual items: blood pressure, cholesterol, glucose, body mass index (BMI), smoking, physical activity, and diet) from baseline to 6 months post-randomization with intention-to-treat analysis. Secondary outcomes include: change in individual LS7 items, quality of life; stroke awareness, adverse events; health service use; and costs. Based on pilot data, 790 participants (395 IG, 395 UCG) will be required to provide 80% power (two-sided α = 0.05) to detect a mean difference in the LS7 of ⩾0.40 (SD 1.61) in IG compared to 0.01 (SD 1.44) in the UCG at 6 months post-randomization. Discussion: Stroke is largely preventable. This study will provide evidence of the effectiveness of a mobile app to reduce stroke risk. Trial registration: ACTRN12621000211864.