Optical fiber sensors are recognized as a promising technology for detecting parameters such as temperature, biomolecules, and chemical substances. Among these, multimode interference (MMI) sensors stand out for their high sensitivity, ease of fabrication, and low cost. This work presents the design and analysis of an interferometric sensor based on a single-mode–multimode–single-mode (SMS) structure, in which the multimode section consists of a coreless fiber whose diameter was reduced from 125 µm to 20 µm. Numerical simulations using FIMMWAVE software were performed for external refractive indices (RI) between 1.33 and 1.43, evaluating sensitivity in two spectral ranges (600–800 nm and 900–1100 nm) and achieving a maximum value of 918.21 nm/RIU for the smallest diameter. The influence of the MMF length on resonance peak position and spectral selectivity was also studied. Experimental validation was carried out with a 125 µm coreless MMF of ≈15 mm length, using solutions of different refractive indices. The experimental results confirmed the sensor’s effective RI response and demonstrated the feasibility of the proposed configuration as a basis for developing low-cost, high-precision optical biosensors.