<p>Bi₂Se₃ is a classic n-type mid-low temperature thermoelectric material limited by high lattice thermal conductivity and poor mechanical properties. Herein, dense Bi_2-xIn_xSe₃ bulks (x = 0–0.02) were fabricated via melt-compaction and hot pressing. Structural and XPS characterizations verify uniform In substitution without impurity phases, and partial reduction of In³⁺ to In⁺ driven by intrinsic Se vacancies generates In⁺–V_Se composite defects. Low In doping introduces isolated Se vacancies to suppress carrier mobility, while moderate doping passivates ionized scattering centers and partially recovers mobility. All In-doped samples exhibit reduced power factor due to the trade-off between conductivity and Seebeck coefficient. Nevertheless, lattice distortion, mass/strain fluctuations, and composite defects drastically scatter phonons and cut lattice thermal conductivity. The optimized Bi_1.985In_0.015Se₃ achieves a peak ZT of 0.43 at 573&#xa0;K, 23% higher than pristine Bi₂Se₃. Vickers indentation tests confirm enhanced hardness and fracture toughness after In doping, favoring device fabrication and service stability. This work clarifies the dual carrier-phonon modulation mechanism of mixed-valence In doping and offers a defect engineering strategy to simultaneously improve thermoelectric and mechanical performances of Bi₂Se₃.</p>