ThisSurface plasmon resonance waveguide sensor chapter presents the analysis of a surface plasmon resonance (SPR) waveguide sensor with a thin InSb layer in the sensing section in the terahertz region. First, the eigenmode analysis of the sensing section is performed using water as the analyte. Then, the eigenmodes are calculated using the Yee-mesh based imaginary distance beam propagation method. As a result, the eigenmodes of the surface plasmon polariton exist in the sensing region, and the maximum SPR is expected to be around 1.28 THz at 300 K. Next, we focus on the treatment of the InSb-water interface and investigate the performance of the sensor using the frequency-dependent FDTD method based on the simple trapezoidal recursive convolution technique. As expected, the SPR absorption is found to be maximum at 1.26 THz. Furthermore, the SPR response is investigated and found to be capable of detecting temperature variations from 280 to 320 K. As an application, the possibility of detecting mixtures of organic solvents and water is investigated. The sensor with an analyte container is also investigated. A steep frequency response is achieved with a low height incidence waveguide. Finally, a practical three-dimensional sensor is also investigated.