- Organic–inorganic nanocomposite materials combine the advantages of organic polymers and inorganic nanoparticles. Due to their high application potential, the investigation of these materials is of great interest. In this study we examine the dielectric response of CoFe2O4@PU nanocomposites subjected to an external electric field. Dielectric properties were investigated in a frequency range of 10–2 Hz to 1 MHz and over a temperature range of 213 to 333 K, while varying the content of ferromagnetic particles. The experimental results provide insight to the structure–property relationship of nanocomposites subjected to an alternating electric field. We discuss two relaxation processes (β- and α-processes) that show a dependence on the particle content. For the β-process we observe an Arrhenius-like behavior with temperature, and the related activation energies decrease with increasing particle content. Evidently, the particles' presence facilitates the β-process relaxation in the polymer matrix, resulting in higher dielectric losses. The α-process is attributed to a glass transition of the matrix, and the corresponding transition temperature Tg is compared to Tg obtained from differential scanning calorimetry (DSC).