The emergence of structured optical waveforms as precisely engineered photonic frameworks has remarkably propelled the mastery of electromagnetic field distributions, thereby fortifying the light-matter interaction essential for high-performance fiber-optic sensing. In this work, a Tricomi-Gauss beam, characterized by its propagation-invariant behavior, is employed as the excitation source within a decladded multimode fiber to overcome the inherent limitations of conventional Gaussian illumination, which is typically constrained by limited evanescent-field penetration and weaker modal-medium interaction. The key novelty lies in leveraging a Tricomi-Gauss beam to enable selective higher-order mode excitation in a decladded multimode fiber without requiring any structural modification to the waveguide dimensions, thereby augmenting the modal-field interaction with the sensing medium, which leads to intensified evanescent-field strength and enhanced sensitivity to refractive-index variations. This novel sensing configuration is rigorously analyzed using a beam propagation method implemented in OptiBPM, with particular emphasis on modal evolution across the sensing region. Our proposed sensor manifests an exceptional peak absolute sensitivity of 3352.10 dB/RIU, accompanied by a notably refined detection resolution of 2.98×10−6RIU. Therefore, these observations demonstrate the pronounced superiority of the proposed sensor over the Gaussian-based excitation methodology and affirm that structured beam engineering enables coherent, systematic sensitivity amplification without structural intervention in the fiber, making it suitable for next-generation high-performance refractometric sensing applications.