Background: Many patients suffering from post-acute COVID-19 syndrome must deal with fatigue. They need physical and psychological support, strengthening, and adaption of their individual lifestyle. The use of apps can foster fatigue management. Objectives: The aim of this study was to analyse experiences of patients with fatigue caused by post-acute COVID-19 syndrome after using Untire for more than two weeks. Methods: A qualitative research design was chosen to identify user-experience after using the app. Seven semi-structured interviews were conducted and qualitative content analysis according to Mayring was applied. Results: The Untire App was generally perceived as being easy to use. Patients judged the Untire App as supportive in most of the features, especially in energy measurement and relaxation exercises. Conclusion: Thus, though developed for cancer patients, this mobile health (mHealth) application is with some restrictions also suitable for patients with post-acute COVID-19 syndrome.