Metallically conducting flexible alginate fibers with superior mechanical strength and environmental stability was obtained by metal ion-exchange using a simply modified continuous wet-spinning process. Spinning solution was prepared by dissolving Na-alginate polymer in distilled water, followed by spinning into CaCl2 coagulation bath to form Ca-alginate swollen gel fiber. The Ca-alginate fiber was then immersed into AgNO3 aqueous solution for incorporation of Ag+ ion into the swollen Ca-alginate alginate fiber, resulting in the alginate fiber complexed with both Ca++ and Ag+ ions (Ag/Ca-alginate fiber). The Ag/Ca-alginate fiber was finally immersed into aqueous dimethylamine borane complex (DMAB) reduction solution, reducing Ag+ ions to Ag nanoparticles (AgNP) in the fiber. The AgNP embedded Ca-alginate conducting fiber (AgNP/Ca-alginate fiber) was washed with distilled water several times and dried. It was observed that AgNPs were uniformly formed and dispersed both on the surface and inside of the AgNP/Ca-alginate fiber. The fiber exhibits superior room temperature electrical conductivity and mechanical strength as high as 2,000 S/cm and 290 MPa, respectively. The AgNP/ Ca-alginate fiber also possessed excellent environmental stability, showing little conductivity change even after 800 hours under a harsh 60 oC and 70% relative humidity condition. Conductive fabric with extremely low surface resistivity of 0.6 Ω/□ could be fabricated using the AgNP/Ca-alginate fibers.