The three-dimensional structure of canecystatin-1, a potent inhibitor of cysteine proteases from sugarcane (Saccharum officinarum), has been solved in two different crystal forms. In both cases, it is seen to exist as a domain-swapped dimer, the first such observation for a cystatin of plant origin. Size exclusion chromatography and multidimensional NMR spectroscopy show the dimer to be the dominant species in solution, despite the presence of a measurable quantity of monomer undergoing slow exchange. The latter is believed to be the active species, whereas the domain-swapped dimer is presumably inactive, as its first inhibitory loop has been extended to form part of a long β-strand that forms a double-helical coiled coil with its partner from the other monomer. A similar structure is observed in human cystatin C, but the spatial disposition of the two lobes of the dimer is rather different. Dimerization is presumably a mechanism by which canecystatin-1 can be kept inactive within the plant, avoiding the inhibition of endogenous proteases. The structure described here provides a platform for the rational design of specific cysteine protease inhibitors for biotechnological applications. DATABASE: The coordinates and structure factors have been deposited in the Protein Data Bank under the accession codes 3UL5 and 3UL6. STRUCTURED DIGITAL ABSTRACT: (Figure is included in full-text article.)