Dissimilar fusion welding between NiTi shape memory alloy and AlCoCrFeNi 2.1 , eutectic high entropy alloy was achieved using a combination of Nb and Cu interlayers. Both base metals exhibit unique properties that complement each other making them attractive for integration in smart systems. However, direct joining of these materials is not feasible due to the formation of brittle intermetallic compounds that compromise the joint integrity. Thus, to avoid this metallurgical issue dissolution of the Nb and Cu interlayers promoted elemental intermixing and altered solidification pathways, leading to the formation and intensification of topologically close-packed phases. Phase evolution was assessed through thermodynamic CALPHAD simulations and validated using synchrotron X-ray diffraction, while further microstructural characterization employed backscattered electron imaging and energy dispersive spectroscopy. The weld exhibited pronounced spatial heterogeneity, with a peak hardness of 856 HV 0.3 near the NiTi-side interfacial region, consistent with enrichment of TCP phases and Ti-rich intermetallics. Tensile testing showed fracture at an ultimate tensile strength of 372 MPa and a fracture strain of 1.11%, with crack initiation in the hardened interfacial region followed by mixed-mode fracture toward the fusion zone. Although tensile ductility remains limited, the multi-interlayer strategy enabled defect-free joint formation and confined embrittlement to a narrow region compared with direct joining. • Fusion welding of NiTi shape memory alloy with AlCoCrFeNi 2.1 eutectic high entropy alloy is investigated using multiple interlayers to minimize intermetallic formation. • Interlayer dissolution produced a heterogeneous microstructure, promoting the formation of topologically close-packed phases that influenced the mechanical behaviour of the joint. • CALPHAD simulations and synchrotron X-ray diffraction revealed the impact of elemental intermixing and phase evolution across the composite weld zone.