<p>This study investigates the influence of Bi³⁺ substitution on the structural, magnetic, and electrical properties of polycrystalline Sm_0.55-xBi_xSr_0.45MnO₃ (0 ≤ x ≤ 0.1) polycrystalline manganites synthesized via the conventional solid-state reaction method. Rietveld refinement of X-ray diffraction patterns confirms that all compositions crystallize in a single-phase orthorhombic perovskite structure, accompanied by a gradual lattice expansion with increasing Bi content. Scanning electron microscopy (SEM) reveals well-connected grains that exhibit a modest enlargement at higher Bi concentrations. Temperature-dependent resistivity measurements demonstrate a metal–insulator transition that shifts from 131&#xa0;K (x = 0) to 120&#xa0;K (x = 0.1), with pronounced thermal hysteresis between heating and cooling cycles, indicative of the first-order character of the transition. Magnetization measurements performed under an applied field of 1000 Oe show a systematic reduction in the Curie temperature from 138 to 123&#xa0;K, without any bifurcation between the zero-field-cooled and field-cooled curves. Curie–Weiss analysis in the paramagnetic regime yields positive Weiss temperatures and decreasing effective magnetic moments, consistent with a weakening of the Mn³⁺-O-Mn⁴⁺ double-exchange interactions. Notably, Bi substitution enhances the low-field magnetoresistance, reaching a maximum of 91% for x = 0.1, which correlates with improved spin alignment. Collectively, these findings demonstrate that Bi doping offers an effective strategy to tune the interplay among structural, magnetic, and transport properties in narrow-bandwidth Sm-based manganites.</p>