Noncatalytic carbohydrate binding modules (CBMs) are components of glycoside hydrolases that attack generally inaccessible substrates. CBMs mediate a two- to fivefold elevation in the activity of endo-acting enzymes, likely through increasing the concentration of the appended enzymes in the vicinity of the substrate. The function of CBMs appended to exo-acting glycoside hydrolases is unclear because their typical endo-binding mode would not fulfill a targeting role. Here we show that the Bacillus subtilis exo-acting β-fructosidase SacC, which specifically hydrolyses levan, contains the founding member of CBM family 66 (CBM66). The SacC-derived CBM66 ( Bs CBM66) targets the terminal fructosides of the major fructans found in nature. The crystal structure of Bs CBM66 in complex with ligands reveals extensive interactions with the terminal fructose moiety (Fru-3) of levantriose but only limited hydrophobic contacts with Fru-2, explaining why the CBM displays broad specificity. Removal of Bs CBM66 from SacC results in a ∼100-fold reduction in activity against levan. The truncated enzyme functions as a nonspecific β-fructosidase displaying similar activity against β-2,1– and β-2,6–linked fructans and their respective fructooligosaccharides. Conversely, appending Bs CBM66 to BT3082, a nonspecific β-fructosidase from Bacteroides thetaiotaomicron , confers exolevanase activity on the enzyme. We propose that Bs CBM66 confers specificity for levan, a branched fructan, through an “avidity” mechanism in which the CBM and the catalytic module target the termini of different branches of the same polysaccharide molecule. This report identifies a unique mechanism by which CBMs modulate enzyme function, and shows how specificity can be tailored by integrating nonspecific catalytic and binding modules into a single enzyme.