Abstract: To assist with smoking cessation, wearable devices are used to detect the puff (hand-to-mouth gesture) recognition within the smoking activity in a ubiquitous manner. There is a strong assumption that smoking and wearing a smartwatch are usually with the same hand. It will certainly fail to detect smoking gesture with the opposite hand. In this work, we find an interesting phenomenon: smoking can cause a unique pattern of heart rate (HR) which is quite different from other daily activities’ effects. Based on this psychophysiological response, we propose HeartIt, a just-in-time smoking detection solution through measuring the HR by a smartwatch. HeartIt works well for the smoker wearing a smartwatch on either wrist. It can accurately distinguish smoking from other similar hand-to-mouth gestures (e.g., eating, drinking). Moreover, we design an adaptive tracker to trigger the HR sensor once the gesture of lighting a cigarette is detected by low-cost accelerometers. It is robust for different people in various postures and scenarios. Our real-world experiments show that the precision and recall rate of HeartIt reaches 96.7% and 99.8%, respectively.