Bacterial disulfide isomerases are typically homo-dimers, which shuffle incorrect disulfide bonds in their protein targets. These enzymes are essential for the correct folding of proteins required for bacterial virulence and survival under stress. Proteus mirabilis ScsC (PmScsC) is a disulfide isomerase that surprisingly, functions as a trimer. We have solved the crystal structure of PmScsC in three different conformations. The major conformational difference observed in these structures is the distance between the trimerisation domain and the catalytic domain. In the compact structure the catalytic domains of all three protomers pack close to the trimerisation domain, in the extended structure the catalytic domains are much further away from the trimerisation domain, while the transitional structure has features of both other structures. Comparison of these structures reveals a flexible linker in the protein, which changes secondary structure to facilitate the conformational changes. The structures also show that P. mirabilis ScsC has a large range of motion and we predict that this is necessary for its disulfide bond shuffling activity.