With the rapid advancement of soft electronics, particularly the rise of fiber electronics and smart textiles, there is an urgent need to develop high-performance fiber materials with both excellent electrical and mechanical properties. However, existing fiber materials including metal fibers, carbon-based fibers, intrinsically conductive polymer fibers, and composite fibers struggle to simultaneously meet the requirements. Here, we introduce a metalgel fiber with a unique structure. In metalgel fibers, liquid metal forms a continuum, extending throughout the entire volume of nanostructured fucoidan polymer networks, which are immobilized by electrostatic interactions. The distinctive structure imparts the fiber with metallic conductivity (2.8 × 106 S·m-1), softness (Young's modulus of 1.8 MPa), and stable electromechanical coupling properties (resistance change <5% after 20,000 times pressing, stretching, bending, and twisting cycles). The metalgel fibers were woven into multifunctional smart textiles, highlighting their potential for practical applications.