Radicals are key intermediates in numerous reactions. Their high reactivity enables various transformations to occur under mild conditions, however, also brings great challenges to control their reactions especially over heterogeneous catalysts. Here we propose to use metal nanoparticles to directly steer the conversion of free radical species. Results from photocatalytic reactions, in situ transient absorption spectroscopy, and theoretical simulation demonstrate that supported Pd nanoparticles can efficiently stabilize free radical species generated from photo-excited TiO₂, and thus manipulate their conversion on catalyst surface, owing to the enhanced electronic interactions between metal and radical species. These understandings are crucial for the design of advanced heterogeneous catalytic systems with controllable radical reactions.