Abstract: Twinning-induced plasticity (TWIP) steel was processed using electrically assisted friction stir welding (EFSW). The microstructure, mechanical properties, and deformation behavior of the welded joints were systematically investigated. The results show that the average grain size was refined from 3.67 μm in the base material (BM) to 1.39 μm in the stir zone (SZ), while it increased to 4.19 μm in the heat-affected zone (HAZ). The fraction of twin boundaries (TBs) decreased from 20.7% in the BM to 6.9% in the SZ and increased to 24.5% in the HAZ. The ultimate tensile strength, yield strength, and elongation of the BM were 1021 MPa, 505 MPa, and 65.8%, respectively. In comparison, the EFSW joint exhibited values of 1055 MPa, 561 MPa, and 60.8%, corresponding to 103.3%, 111.1%, and 92.4% of those of the BM, respectively. During tensile testing, plastic deformation was primarily concentrated in the BM, although both the SZ and HAZ also exhibited notable plastic deformation. Fracture ultimately occurred in the BM.