Inerter-based Vibration Absorbers (IVAs) have gained extensive attention in research in vibration control due to their high performance and light weight. In practice, the optimal tuning parameters of IVAs are determined by structural dynamic properties of the primary structure for an optimal control performance. However, the structural dynamic properties (such as natural frequency) may deviate from the design values, which leads to mistuning of IVAs, thus weakening the control performance. In order to address this issue, an in-situ adjustable IVA with a rhombus amplification mechanism (R-IVA) is proposed to compensate for the mistuning in practice. Firstly, the theoretical model of the proposed R-IVA was established. Subsequently, based on H∞ optimization, closed-form solutions for the optimal parameters of the proposed R-IVA are derived. Parametric studies are performed to explore the influence of essential parameters of the R-IVAs. Finally, the applicability of R-IVA and the theoretical optimization solution is illustrated by numerical analysis on prototype structures under seismic excitations. Conclusively, with the rhombus amplification mechanism, in-situ adjustable inertance can be achieved by tuning the assembly angle. The mistuning problem in practice can be effectively rectified by the proposed R-IVA device and optimization method.