We present ShadowTouch, a novel sensing method to recognize the subtle hand-to-surface touch state for independent fingers based on optical auxiliary. ShadowTouch mounts a forward-facing light source on the user’s wrist to construct shadows on the surface in front of the fingers when the corresponding fingers are close to the surface. With such an optical design, the subtle vertical movements of near-surface fingers are magnified and turned to shadow features cast on the surface, which are recognizable for computer vision algorithms. To efficiently recognize the touch state of each finger, we devised a two-stage CNN-based algorithm that first extracted all the fingertip regions from each frame and then classified the touch state of each region from the cropped consecutive frames. Evaluations showed our touch state detection algorithm achieved a recognition accuracy of 99.1% and an F-1 score of 96.8% in the leave-one-out cross-user evaluation setting. We further outlined the hand-to-surface interaction space enabled by ShadowTouch’s sensing capability from the aspects of touch-based interaction, stroke-based interaction, and out-of-surface information and developed four application prototypes to showcase ShadowTouch’s interaction potential. The usability evaluation study showed the advantages of ShadowTouch over threshold-based techniques in aspects of lower mental demand, lower effort, lower frustration, more willing to use, easier to use, better integrity, and higher confidence.