This work demonstrates a facile route to the synthesis of large quantities of uniform polyaniline-sodium alginate (PANI-SA) nanofibers template-guided by SA. This approach is an easy, inexpensive, environmentally friendly, and scalable one-step method to produce uniform nanofibers with controllable average diameters in bulk quantities. We started with biopolymer-monomer complexes formed between the carboxylic groups of SA and the amino group of an organic monomer (aniline). When ammonium persulfate was added, such polymer-monomer complexes could be polymerized. Then, polyaniline-sodium alginate nanofibers with uniform diameters from 40 to 100 nm were successfully obtained in a high yield. The resultant PANI-SA nanofibers were characterized by means of different techniques, such as ultraviolet-visible spectroscopy, thermogravimetric analysis, wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, and scanning and transmission electron microscopy methods. The mechanism governing the formation of the polyaniline-sodium alginate nanofibers is discussed.