The impedance matching concept is employed in this paper to select the optimum gain of a VSC-HVDC damping controller. The damping controller of the DC link is based on control of active power in proportion to the difference in local frequency at the VSC-HVDC converter stations. To explain impedance matching, the small-disturbance electro-mechanical dynamics of a power system is transformed to an equivalent LC-circuit. The VSC-HVDC damping controller corresponds to introducing a resistor in the circuit model. Using impedance matching to select resistor value in the circuit model is equivalent to selecting damping controller gain that gives maximum damping ratio. An analytical derivation is conducted for a linearized model of a generic two-area power system, including an expression for the optimum gain. It is also shown how the concept may be employed without a circuit model and that the optimum value is hardly affected by changes in power flow. Then the performance of the proposed approach in multi-mode test systems and with nonlinearities is evaluated using dynamic simulations in DIgSILENT PowerFactory. The results show that impedance matching maximizes the targeted mode damping ratio while non-targeted modes are not negatively impacted.