2S-soy protein, a biopolymer extracted from soy protein isolate (2S-SPI), was studied as a non-covalent surfactant for polymer nanocomposites. This study showed that 2S-SPI effectively improved carbon nanofibers (CNFs) dispersion in poly(vinylidene fluoride). 2S-SPI surfactant had remarkable impact on both electrical conduction and dielectric relaxation of the nanocomposites, particularly, at high temperatures. 2S-SPI modified CNFs caused coupling of conductivity relaxation and structural relaxations of the nanocomposites, in contrast to pristine CNFs. Both Maxwell–Wagner–Sillars and conductivity relaxations were enhanced at high temperatures by 2S-SPI, which made different contributions to electrical conduction of the nanocomposites with or without surface modification.