Two samples of acid-hydrolyzed nanocellulose and two samples of TEMPO-oxidized nanocellulose were separately prepared from cotton liner pulp and microcrystalline cellulose, and dispersed in water. Sodium alginate that was extracted from brown seaweed was dissolved in the nanocellulose suspensions and wet spun in a calcium chloride bath to form four kinds of alginate/nanocellulose composite fibers. The structures and properties of the obtained nanocellulose and composite fibers were investigated and compared. The results showed that all of the nanocellulose samples exhibited a needle shape with slightly different sizes. The incorporation of nanocellulose increased the opacity of the spinning dopes but improved the mechanical properties of the alginate fibers. The optimum addition amount for all of the nanocelluloses was 5% (based on the weight of sodium alginate). The TEMPO-oxidized nanocellulose produced from cotton liner pulp had the greatest influence on the strength of the fibers. All the composite fibers had an irregular cross-section with dense and uniform structure, which indicated the good compatibility between nanocellulose and alginate. In addition, the introduction of nanocellulose slightly improved the thermal stability of the alginate fibers.