The development of biopolymer fibers is attracting considerable interest due to the need to reduce the environmental impact of the petroleum-based industry. With the aim to foster the use of biopolymer-based antimicrobial fibers, a green and sustainable preparation route for alginate fibers containing copper-based nanostructures is reported. Its strong antimicrobial properties and affinity for alginate make copper the ideal candidate for the preparation of products suitable for various applications. In this work alginate is extruded and crosslinked in a Cu2+ aqueous bath, producing a filiform hydrogel structure. Ascorbic acid was then used to reduce the metal ions and to form the aforementioned nanostructures. This in situ strategy for the reduction of coordinating Cu2+ ions is unprecedented and leads to a homogeneous distribution of the inorganic structures in the polymeric network. The entire process can be monitored through infrared spectroscopy and the performances (e.g., thermal stability, morphology, swelling, water retention, mechanical properties) of the obtained products are tunable as a function of the duration of each preparation step. The great affinity for water and the small amount of Cu2+ released as a function of time suggest promising perspectives for the use of these fibers in antimicrobial applications. Display omitted •Cu2+-alginate fibers obtained through a facile extrusion and crosslinking process.•Ascorbic acid to reduce Cu2+ ions in situ.•Fibers decorated with homogeneously distributed copper-based nanostructures.•Properties of the fibers finely tuned as a function of the preparation conditions.