An ultrahigh sensitivity polarimetric strain sensor is proposed based upon a four-layer D-shaped optical fiber and surface plasmon resonance (SPR) technology. In contrast to existing SPR-based sensors, which are based on changes in the refractive index of the overlayer, the sensor proposed in this study is based on the change in the refractive index of the fiber core in response to the application of an axial load. Specifically, the phase difference between the P and S waves after passing through the sensor under SPR conditions is measured using a common-path heterodyne interferometer and is used to determine the corresponding change in the refractive index of the core, from which the strain is then inversely derived. The experimental results show that the sensitivity of the proposed sensor is around 2.19×10⁴ deg/ε, i.e., degree/strain. By contrast, that of a conventional (non-SPR) polarimetric fiber sensor is just 5.2×10² deg/ε. To the best of the authors' knowledge, the sensor proposed in this study represents the first reported attempt to exploit the refractive index change of the core of an SPR-based fiber sensor for strain measurement purposes.