We investigated microstructural deformations and Bi segregation in GaAs/GaAsBi/GaAs core–multishell heterostructures, which were triggered by the existence of twin defects. We observed Bi segregation at the interface of the twin defect interface in the GaAsBi shell. The phenomenon produced a horizontally spread Bi-accumulated nanostructure in the nanowire, which is probably induced by the large lattice mismatch between GaAs and GaAsBi. Bi is expected to penetrate through the twin defect interface, which results in the existence of Bi along twin defects and also inside the GaAs core. The existence of twin defects induced structural deformations and resulted in the formation of corrugated complex sidewall surfaces on the nanowire.