Jacalin, a tetrameric two-chain lectin (66,000 M_r) from jackfruit seeds, is highly specific for the tumour associated T-antigenic disaccharide. The crystal structure of jacalin with methyl-α-D-galactose reveals that each subunit has a three-fold symmetric β-prism fold made up of three four-stranded β-sheets. The lectin exhibits a novel carbohydrate-binding site involving the N terminus of the α-chain which is generated through a post-translational modification involving proteolysis, the first known instance where such a modification has been used to confer carbohydrate specificity. This new lectin fold may be characteristic of the Moraceae plant family. The structure provides an explanation for the relative affinities of the lectin for galactose derivatives and provides insights into the structural basis of its T-antigen specificity.