Plasmonic sensors offer ultra-sensitive detection capabilities at the nanoscale, enabling the detection of slight variations in refractive index, molecular binding events, and environmental conditions. Their compact size and compatibility with integrated circuits make them promising candidates for various applications, ranging from biomedical diagnostics to environmental monitoring and beyond. In this study, a comprehensive numerical investigation is conducted using the finite element method (FEM) to analyze a plasmonic sensor employing a metal–insulator–metal (MIM) waveguide for temperature sensing applications. The sensor configuration includes a resonant cavity that is coupled to the MIM bus waveguide and filled with a PDMS polymer. Initially, the device's sensitivity is approximately −0.44 nm/°C. While recognizing the existence of various sensitive plasmonic sensor designs, a gap in understanding the light coupling mechanisms to nanoscale waveguides is identified. To address this, a novel approach is employed: orthogonal mode couplers specifically tailored for plasmonic chips utilizing MIM waveguide-based sensors . Through optimization, the hybrid system, comprising silicon couplers and an MIM waveguide, exhibits optimized transmission ranging from −1.73 dB to −2.93 dB across a broad wavelength spectrum of 1450–1650 nm. The strategic integration of these couplers not only distinguishes the proposed plasmonic sensor but also positions it as a highly promising solution for an extensive range of sensing applications.