Due to their potential biomedical applications, protein-based hydrogels have attracted considerable interest. Although various methods have been developed to engineer self-assembling, physically-crosslinked protein hydrogels, exploring novel driving forces to engineer such hydrogels remains challenging. Protein fragment reconstitution, also known as fragment complementation, is a self-assembling mechanism by which protein fragments can reconstitute the folded conformation of the native protein when split into two halves. Although it has been used in biophysical studies and bioassays, fragment reconstitution has not been explored for hydrogel construction. Using a small protein GL5 as a model, which is capable of fragment reconstitution to reconstitute the folded GL5 spontaneously when split into two halves, GN and GC, we demonstrate that protein fragment reconstitution is a novel driving force for engineering self-assembling reversible protein hydrogels. Fragment reconstitution between GN and GC crosslinks GN and GC-containing proteins into self-assembling reversible protein hydrogels. These novel hydrogels show temperature-dependent reversible sol-gel transition, and excellent property against erosion in water. Since many proteins can undergo fragment reconstitution, we anticipate that such fragment reconstitution may offer a general driving force for engineering protein hydrogels from a variety of proteins, and thus significantly expanding the 'toolbox' currently available in the field of biomaterials.