The use of mobile applications in chronic disease management has grown significantly over the past decade. When properly designed, these apps provide a convenient, safe, high-quality service to patients. In this study, a health management app was developed, and its usability among patients with diabetes mellitus was examined. A convenience sample of 136 patients, referred to two academic centers from December 2016 to July 2017, was enrolled. Upon completion of informed consent, the participants were asked to install the app on their smartphone. Two weeks later, they were required to complete a postapplication usability questionnaire, comprising 21 questions classified into six domains. The response rate was 89%. The highest scores were given for "ease of use and learnability"; the lowest-scored domains were "interaction quality" and "reliability." Urban residents, participants with lower educational qualifications, and retirees were significantly more satisfied with the app. Overall, patients with diabetes mellitus perceived the app as useful for disease management. However, the overall usability of health apps is expected to improve when a multidisciplinary team (health professionals, computer engineers, art designers) is involved in the development process.