In this study, a plasmonic biosensor based on a metal–insulator–metal (MIM) waveguide is proposed for high-sensitivity refractive index detection and analyzed numerically. The design consists of a hybrid configuration comprising a pentagonal resonant cavity coupled with a narrow rectangular slot located at its center. This specific arrangement enables the excitation of two distinct resonance modes, leading to improved spectral resolution and sensing accuracy compared to conventional MIM structures. The finite-difference time-domain (FDTD) simulation method was employed to investigate the optical response and optimize the geometric parameters of the design. The optimized sensor, with a sensitivity of 1272 nm/RIU and a figure of merit (FOM) of 7930 RIU-1, demonstrates excellent sensing capability and strong electromagnetic field confinement. Owing to its compact geometry and compatibility with nanoimprint lithography, the proposed design is highly manufacturable. The conducted analyses indicate that this structure possesses the ability to detect and distinguish between Vibrio cholerae, Escherichia coli, and Pseudomonas bacteria.