In this research paper, we propose a refractive index (RI) based plasmonic nanosensor comprising of metal-insulator-metal (MIM) waveguide and octagonal resonator embedded with gold nanorods. With the help of the finite element method (FEM), spectral characteristics are analyzed to determine the detection ability of the sensor in different wavelengths of the mid-infrared (MIR) range. By modifying important parameters and following a linear relationship between the transmittance profile and changes in the refractive index, the functionality of refractive index sensing is also studied. After optimization, the optimal structure achieved the sensitivity of 13,157 nm/RIU (Refractive Index Unit), which is the highest sensitivity recorded in plasmonic MIM waveguide-based sensors, as well as a high dip strength of 0.675921. Using gold nanorods, the plasmonic sensor can increase the sensitivity by 65.51%. The sensing resolution of the sensor is 7.6 × 10−8 for dielectrics with refractive indices from 1.32 to 1.38. Furthermore, the proposed sensor can be used as a nanophotonic device. Also, the refractive index detection range is excellent for different liquids (e.g., seawater, distilled water, water, mineral water, air, and optic oil). The designed structure is simple and easy to fabricate, so it can be used in nanometer-scale refractive index sensors. It can be a potential alternative to silver-based sensors due to its chemical stability and satisfactory performance in refractive index and biosensing.