A novel mechatronic semiactive inerter (MSAI) by using an operational amplifier (OPA) circuit-based adjustable capacitor is proposed in this study to address the realization challenge of fast-responding semiactive inerters. The fast-responding ability, inertance-directly controlled capability, and sensor-less regulation characteristic are distinctive advantages of the proposed MSAI. To implement a pure controlled capacitor and a pure capacitive motor load (the internal resistance of the motor is canceled), two different types of OPA circuits are designed. Tuned inerter damper and tuned viscous mass damper, as two typical MSAI-based configurations, are established and tested. It is found that the proposed MSAI can modify the inertance quickly (in the microsecond time scale) and reliably, and good congruence between theoretical and experimental results is obtained.