Background: The number of people with dementia is increasing worldwide and to date there is no curative treatment, making it one of the biggest public health challenges for the upcoming decades. Yet, health- and lifestyle factors account for a substantial part of all dementia cases worldwide, which opens up the opportunity of primary prevention of dementia. The general public is still largely unaware of the potential of dementia risk reduction. mHealth interventions can potentially contribute to improving knowledge and motivation for dementia risk reduction in a low-cost and scalable way. Objectives: To evaluate two versions (standard and extended) of the mobile app “MijnBreincoach” (“MyBraincoach”) on usage pattern, appreciation, and beliefs and attitudes related to dementia risk behaviors. Both versions aimed at improving knowledge and identifying dementia risk behavior. The extended version also aimed at increasing beliefs and attitudes related to identified dementia risk behaviors. Methods: A pre-post-design was used to assess usage pattern (usage-tracking data), appreciation, and seven constructs of beliefs and attitudes related to dementia risk reduction (the MCLHB-DRR Scale). Participants were community-dwelling middle-aged adults (40-75 years) from the Netherlands. Participants used one of the two versions of the app. Linear mixed models were used, adjusted for age, sex and educational level. Results: Of all participants (n=299), 190 (63.6%) completed the post-evaluation and 167 (55.9%) had installed the app. The most reported reason for non-use was technical problems during installation and use (47.1%). Women installed the app more often than men (63.1% vs. 48.7%; ꭓ2 (1) = 6.304, P=.01). Of all participants who responded to the follow-up assessment and reported to have used the app (n=102), 78.4% stated that the app provided a positive approach towards brain health and >80% felt better informed about dementia risk reduction. Usage-tracking showed that only 16 of the 101 participants (15.8%) who were randomized to the extended app used the main additional feature. An intention-to-treat analyses revealed no pre-post changes in beliefs and attitudes (perceived susceptibility B -0.07, P=.56; perceived severity B -0.26, P=.18, perceived benefits B -0.13, P=.20, perceived barriers B 0.23, P.16, cues to action B 0.09, P .60, general health motivation B 0.01, P .92, and self-efficacy B-0.05, P .62). No significant differences were found between the two versions of the app. Female, younger (<60y) and low-to-middle educated participants evaluated the app most positively. Conclusions: Most participants evaluated the app positively and stated to have increased their knowledge on the topic. Usage of the extended functions of the app was low, which may explain why modification of beliefs and attitudes towards behavioral change did not change. Improving the use of the app is important to take into consideration in future studies.