Alginate is a well known environmentally friendly material which has found many applications such as the preparation of dressings to treat exuding wounds, drug delivery, enzyme immobilization, etc.; however its use is limited due to low mechanical properties. Cellulose nanocrystals (CNC) isolated from cotton were added to calcium alginate fibers with the goal of improving its strength (tenacity), and modulus. The nanocrystals are elongated particles, with an average length of 130nm, an average width of 20.4nm, and an average height of 6.8nm; they were mixed with an aqueous sodium alginate dope solution, and wet spun into a CaCl2 bath to form fibers. It was found that if the apparent jet stretch is kept constant, addition of the nanocrystals reduce the tenacity and modulus of the material; however a small concentration of CNC in the dope solution allow to increase the apparent jet stretch during processing. Characterization of the fibers produced at the maximum apparent jet stretch showed an increase of 38% in tenacity, and 123% in modulus when the CNC concentration was 10%wt. The unusual behavior of the mechanical properties at constant jet stretch may be explained by the orientation of the nanocrystals inside the fiber. A good dispersion of nanocrystals in the matrix is expected due to similarities in cellulose and alginate chemical structure. CNC have potential to become a green, biocompatible, and cost effective solution to reinforce alginate fibers, as well as other biopolymers.