Interfacing blood with any artificial surface instantly triggers the activation of coagulation at the interface, posing a risk of adverse thromboembolic complications. Immediately upon contact, blood proteins adsorb to the surface turning it into an active prothrombogenic and proinflammatory interface promoting the activation of platelets and the coagulation cascade. Here, an interactive hemocompatible nanocoating is designed that synergistically combines antifouling polymer brushes with the activity of an anti‐FXIIa antibody to tackle the two main causes of surface‐induced coagulation. The brushes provide a passive layer rendering the surface stealth and non‐thrombogenic against blood activation. Furthermore, as soon as traces of FXIIa arise, the coating immediately and specifically scavenges it, thus actively switching‐off contact activation right at the beginning. Moreover, a means is developed to translate this coating to a broad range of polymeric surfaces commonly used in medical devices. The coated surfaces prevent protein adsorption and provide a barrier to bacterial colonization while being capable of capturing biologically relevant concentrations of FXIIa. Notably, the coating completely prevents the formation of clots on the surface when exposed to blood. Thus, the interactive anti‐FXIIa nanocoating provides a new avenue to improve the hemocompatibility of medical devices in a safe and efficient manner.