Complex polysaccharides such as agar, alginate, carrageenan, cellulose, xylan, and chitin are abundantly represented in the marine ecosystem. Degradation of these complex polysaccharides into oligosaccharides is achieved by the synergistic action of carbohydrate active enzymes (CAZymes) that are produced by marine microbes. The CAZymes that act on marine polysaccharides predominantly include enzymes such as agarase, alginate lyase, carrageenase, cellulase, pectinase, and xylanase. Since the mid-1990s, several marine bacteria such as Saccharophagus sp., Microbulbifer sp., Bacillus sp. SYR4, Microbacterium oxydans, and Formosa agariphila have been described that produced diverse types of CAZymes. On the basis of amino acid sequence homology studies, the CAZymes have been predominantly categorized as glycoside hydrolases and polysaccharide lyases. Several families of glycoside hydrolase and polysaccharide lyase demonstrate various blends of catalytic modules and are frequently appended with noncatalytic carbohydrate-binding modules (CBM). These modules enable diverse catalytic mechanisms to produce novel oligosaccharides with various pharmacological and nutraceutical properties. The agar- and seaweed-derived oligosaccharides have been reported to demonstrate antioxidative activities such as scavenging of hydroxyl free radicals and superoxide anion radicals along with lipid peroxidation inhibition. The alginate oligosaccharides are credited with several properties such as stimulation of cytokine secretion by human macrophage, activation of vascular endothelial growth factor–mediated growth, migration of human endothelial cells, increased secretion of tumour necrosis factor-α (TNF-α) from the macrophages, and decreased production of reactive-oxygen species in immune cells. Additionally, the alginate oligosaccharides produced by microbial engineering of alginate pathways has been successfully used for bioethanol production. Further sulfated oligosaccharides produced by carrageenase are known to demonstrate anticoagulation, anti-inflammatory, antithrombosis, and antitumor activities, whereas chitooligosaccharides, chitohexaose, and chitoheptaose produced by chitinases are recognized for their antitumor activities.