Sedentary behavior (SB) raises risks for detrimental consequences and is not compensable by other health behaviors such as moderate-to-vigorous physical activity (PA). In order to understand and mitigate SB, tools for measuring and monitoring SB are essential. While current direct-to-customer wearables focus on PA, the VitaBit focusing on SB was developed and validated. It was tested in a laboratory and in a free-living condition comparing it to direct observation and to a current best-practice device, the ActiGraph, on a minute-by-minute basis. In the laboratory, the VitaBit yielded specificity, and negative predictive rates (NPR) of above 91.2 % for sitting and standing, while sensitivity and precision ranged from 74.6 % to 85.7 %. For walking, all performance values exceeded 97.3 %. In the free-living condition, the device revealed performance of over 72.6 % for sitting with the ActiGraph as criterion. While sensitivity and precision for standing and walking ranged from 48.2 % to 68.7 %, specificity and NPR exceeded 83.9 %. According to the laboratory part, high performance for sitting, standing and walking make the VitaBit eligible for SB monitoring. As the results are not transferrable to daily life activities, a direct observation study in a free-living setting is recommended.