In prior research, reading behavior was predominantly measured using either a question­naire, which is economical and easy to implement but imprecise, or paper-pencil diaries that document reading behavior quite accurately, but which are time consuming and costly. The present study aims to introduce and evaluate a precise and easy to implement mea­sure of reading behavior, namely a reading diary app in which participants can record their reading behavior on a smartphone. To evaluate the development procedure, the first research question asked whether data gathered with the app is of high quality (e.g., reliabil­ity). The second research question asked how reading time recorded via the app is related to reading time assessed via different retrospective questionnaires. n = 31 German university students recorded their reading activities for 14 days. Different approaches were applied to estimate the data quality and reliability and yielded satisfactory results. Participants reported more time spent reading daily on the retrospective questionnaire than when re­cording their reading time using the app. The correlation between reading diary app data and questionnaire data was medium in size. Our findings are discussed in the light of future directions for reading research and the use of ambulatory assessments.