Arabinogalactan is a biopolymer consisting of arabinose and galactose monosaccharides. Two classes of arabinogalctans are found in nature; plant arabinogalactan and microbial arabinogalactan. In microbial arabinogalactan the sugars are furanose configurations and is a major structural component of the mycobacterial cell wall.

Arabinogalactan in plants is a major constituents of many gums, including gum arabic, gum gutti and so on. It occasionally attached to proteins and the resulted proteoglycan functions as signaling molecules betweens cells as well as glue to seal wounded part of plants.

The galactan portion of the microbial arabinogalactan is linear, consisting of approximately 30 units with alternating β-(1-5) and β-(1-6) glycosidic linkages. At three branch points within the galactan chain (believed to be at residues 8, 10 and 12)are attached the arabinan chain, which consist of about 30 residues. The arabinan portion of the polymer is a complex branched structure, usually capped with mycolic acids. The arabinan glycosidic linkages are α-(1-3), α-(1-5), and β-(1-2).