Background: Increasing knowledge about stroke may reduce its burden. We examined the effect of the Stroke RiskometerTM mobile phone application (the App) on stroke knowledge in a randomized controlled trial (RCT). Methods: This was a pre-specified secondary outcome analysis in a phase III, prospective, participant and outcome assessor-blinded, two-arm RCT in Australia and New Zealand. Participants were recruited between 2021 and 2023, aged 35–75 years, with ⩾2 stroke risk factors and no cardiovascular disease history. Participants were randomized after assessment of stroke risk factors and knowledge to the intervention group (IG-received risk factor assessment by email and links to the App), and the usual care group (UCG—received risk factor assessment with links to generic information by email). Stroke knowledge was measured at baseline, 3, 6, and 12 months using six validated questions (total score 0 (low knowledge) to 19 (high knowledge)). We used linear and logistic mixed-effects modeling to assess differences in the level of overall stroke knowledge and domains (description, warning signs, risk factors, management) between IG and UCG at each time point. Effect modification of the intervention with age, sex, level of education, ethnicity, socioeconomic status (SES), and country was assessed. Results: There were 862 participants (mean age: 58.1 years (standard deviation (SD): 10.8), 63.0% female, 61.6% tertiary educated, 73.3% European, and 14.7% most disadvantaged area-level SES) randomized to IG (n = 429) and UCG (n = 433). Dropouts (IG/UCG) were as follows: 7.9%/4.8% at 3 months, 3.0%/1.8% at 6 months, and 13.5%/9.0% at 12 months. The time-IG interaction showed a statistically significantly increased overall stroke knowledge (β = 0.50, 95% confidence interval (CI) = 0.02, 0.97) compared with UCG at 6 months only. The intervention effect was stronger in tertiary educated, non-European, and non-Indigenous ethnic groups, and the least disadvantaged SES group. For domains, IG was more likely to correctly identify stroke risk factors (odds ratio (OR) = 1.92, 95% CI = 1.09, 3.39) at 3 months, compared with UCG. Conclusion: The Stroke RiskometerTM App modestly improved stroke knowledge compared with UCG at 6 months but lacks evidence for retaining knowledge at 12 months. As knowledge can drive behavior change, the App may be a tool to enhance primary stroke prevention. Trial registration: ACTRN12621000211864.