This letter proposes a miniaturized wideband Butler matrix (BM) based on dual-layer half-mode substrate integrated waveguide (HMSIW) structure. By virtue of the excellent features of shorter guided wavelength, flatter in-band phase slope, and more compact size of HMSIW, simple and compact couplers and phase shifter are newly designed for BM. Finally, a BM prototype is designed, fabricated, and measured. Benefiting from the proposed structure, size miniaturization over 90% is achieved in comparison with the traditional single-layer substrate integrated waveguide (SIW) type.