Inaccuracies in data measurement can impair trustworthiness of activity trackers (i.e., wearable fitness devices) and, thus, constitute a usability challenge possibly impairing user acceptance. With the present research, we aim at advancing understanding of perceived trustworthiness of activity tracker measurement and its relevance for user acceptance. N = 79 users of activity trackers were surveyed regarding their daily interaction with the tracker, user experience, and user acceptance. Results indicated a substantial variance in perceived trustworthiness. Many users perceived suboptimal trustworthiness of the tracking of their activity data, indicating potential for optimization. Further, analyses showed that higher perceived trustworthiness of activity tracker measurement was indeed linked to higher user acceptance. The results highlight the potential of enhancing user acceptance of activity trackers by improving perceived trustworthiness of activity measurement.