In modern society, stress has become one of the main issues affecting both mental and physical health. The ability to detect stress can help people to improve their daily life. This work proposes ChillIn, a mobile application designed to track and localize stress in an urban environment. The application is based on a single wearable device and its sensors to track stress within urban environments, correlating the data acquired with geographic locations, and highlighting areas of a city that tend to induce stress. Preliminary tests show that ChillIn can help in understanding which are the urban areas where citizens are more stressed, for example to understand if traffic or transportation modes play a role from this point of view. ChillIn has a small impact on the battery duration of the wearable device and runs on a commercially available smartwatch.