In this paper, a seismic sensor based on the combination of ionic polymer metal composite (IPMC) and ferrofluids is presented. The device consists of a vial, filled with ferrofluid, housing an IPMC cantilever beam sensor. Considering that the behavior (e.g., frequency response) of a beam immersed in a fluid changes with the fluid density, in this paper, a novel methodology is proposed to implement a mechanism allowing for the active tuning of the sensor specifications (such as operating range, frequency behavior, and responsivity). To such aim, the addressed methodology exploits external magnetic fields to modify the density of the ferrofluid in which the IPMC sensor is immersed. A description of the sensing methodology and the realization of the sensor prototype are given along with experimental results confirming the expected behavior of the device. Moreover, a model is presented which can be used to predict the IPMC behavior as a function of the fluid properties.