Two key aspect must be considered for the design of modern footbridges: (i) their sensitivity to human-induced vibrations; and (ii) the influence of the variation of the operational and environmental conditions on their modal properties. One possible option, to guarantee an adequate behavior of these structures under both conditioning factors, is the installation of a control system. Among the different systems, active damping devices have shown a great effectiveness when they are used to control the dynamic response of civil engineering structures under uncertainty conditions. Different design algorithms have been proposed to guarantee that structures, controlled by these damping devices, meet the design requirements without compromising the budget. Among these proposals, the motion-based design method has shown a high performance when it has been implemented to design passive damping devices for footbridges under uncertainty conditions. Herein, this design method has been adapted and further implemented for the robust optimum design of active tuned mass dampers when they are employed to control the human-induced vibrations in slender footbridges. According to this method, the design problem can be transformed into two coupled sub-problems: (i) a multi-objective optimization sub-problem; and (ii) a reliability analysis sub-problem. Thus, the main objective is to find the parameters of the active damping device which guarantee an adequate comfort level without compromising its cost. The compliance of this vibration serviceability limit state is computed via a reliability index (related to the probability of failure), which takes into account the effect of the variation of the operational and environmental conditions on the modal properties of the structure. © 2020 European Association for Structural Dynamics. All rights reserved.