Hybrid organic–inorganic piezoelectrics have attracted attention due to their simple synthesis, mechanical flexibility, and designability, which have promising application potential in flexible sensing and self-powered energy harvesting devices. Although some hybrid piezoelectrics are discovered, most of their structures are limited by the perovskite-type and often contain lead. Herein, the synthesis, structure, and piezoelectric properties of a new hybrid lead-free metal halide, (BTMA)₂CoBr₄ (BTMA = benzyltrimethylammonium) are reported. The experimental and theoretical results demonstrate that this material simply composed of [CoBr₄]²⁻ tetrahedra and BTMA⁺ cations exhibits significant piezoelectricity (d₂₂ = 5.14, d₂₅ = 12.40 pC N⁻¹), low Young's and shear moduli (4.11–17.56 GPa; 1.86–7.91 GPa). Moreover, the (BTMA)₂CoBr₄/PDMS (PDMS = polydimethylsiloxane) composite thin films are fabricated and optimized. The 10% (BTMA)₂CoBr₄/PDMS-based flexible devices show attractive performance in energy harvesting with an open-circuit voltage of 19.70 V, short-circuit current of 4.24 µA, and powder density of 11.72 µW cm⁻², catching up with those of piezoelectric ceramic composites. Meanwhile, these film devices show excellent capability in accurately sensing human body motions, such as finger bending and tapping. This work demonstrates that (BTMA)₂CoBr₄ and related piezoelectric lead-free halides can be promising molecular materials in modern energy and sensing applications.