In enzymology, a cyclomaltodextrin glucanotransferase (also cyclodextrin glycosyl transferase or CGTase for short) (EC 2.4.1.19) is an enzyme that catalyzes the chemical reaction of cyclizing part of a 1,4-alpha-D-glucan molecule through the formation of a 1,4-alpha-D-glucosidic bond.

This peculiar enzyme is capable of catalyzing more than one reaction with the most important being the synthesis of non-reducing cyclic dextrins known as cyclodextrins starting from starch, amylose, and other polysaccharides.

CGTase is an enzyme common to many bacterial species, in particular of the Bacillus genus (e.g. B. circulans, B. macerans and B. stearothermophilus) and Brevibacillus brevis.

CGTase also has a weak hydrolyzing activity which consists in cleaving the longer polysaccharidic chains into shorter fragments.

As of late 2007, 47 structures have been solved for this class of enzymes, with PDB accession codes 1A47, 1CDG, 1CGT, 1CGU, 1CGV, 1CGW, 1CGX, 1CGY, 1CIU, 1CXE, 1CXF, 1CXH, 1CXI, 1CXK, 1CXL, 1CYG, 1D3C, 1D7F, 1DED, 1DTU, 1EO5, 1EO7, 1I75, 1KCK, 1KCL, 1OT1, 1OT2, 1PAM, 1PEZ, 1PJ9, 1TCM, 1UKQ, 1UKS, 1UKT, 1V3J, 1V3K, 1V3L, 1V3M, 2CXG, 2DIJ, 3CGT, 4CGT, 5CGT, 6CGT, 7CGT, 8CGT, and 9CGT.