As climate change is progressing, it is important to find mitigation strategies to reach the Paris Agreement’s long-term temperature goal. The objective is to keep the increase in global average temperature to well below 2 °C above pre-industrial levels. One approach to climate change mitigation is to decrease people’s carbon footprint. This complex problem can be approached with top-down programs and bottom-up methods. One such a bottom-up approach is the app Deedster. This app is designed to educate its users about sustainable behavior choices and encourage the users to try out these new behaviors in so-called deeds to reduce their personal carbon footprint. In this research, an exploratory qualitative approach was taken to gather insights into the suitability of the Deedster app as a tool to change individual behavior towards a more sustainable lifestyle as well as investigating obstacles to acting sustainably. App users were interviewed in the beginning and at the end of a four-week Deedster challenge. The findings suggest that the Deedster app is a suitable tool to encourage behavior change and to educate its users about sustainability topics. Based on the results, the app may be stronger in changing low-cost (less difficult) behavior changes than effortful behavior changes. However, in some cases, also effortful behavior changes were reported.  In addition, the users reported more knowledge about sustainable lifestyle alternatives and opened conversations about the app and their behavior changes with others. This study makes valuable contributions to the field of sustainable behavior changes and proposes the Deedster app as a useful method to engage more people in sustainable lifestyles.