This letter proposes an active damper with a series LC filter for suppressing resonances in an ac power-electronics-based power system. The added series filter capacitor helps to withstand most of the system voltage, hence, allowing a lower rated converter to be used for implementing the active damper. Unlike an active power filter for mitigating low-frequency harmonics, the proposed damper dampens resonances at higher frequencies, whose values are dependent on interactions among multiple grid-connected converters and reactive elements of the system. Its control requirements are, therefore, different, particularly in the low-frequency range, where the series LC filter is predominantly capacitive, rather than the usual inductive characteristics that exist between voltage-source converters and the grid (or load). This low-frequency challenge can fortunately be resolved by the proposed fourth-order resonant controller, in addition to the second-order resonant controller used for resonance damping. Experimental results obtained have confirmed the effectiveness of these controllers, and hence, the feasibility of the active damper.