We developed an optogenetically engineered exosome system EXPLORs (exosomes for protein loading via optically reversible protein-protein interactions) that can deliver soluble proteins into the cytosol via controlled, reversible protein-protein interactions (PPI). We induced a transient docking of CRY2-conjugated cargo proteins to the exosomes by introduction CIBN-conjugated with an exosome-associated tetraspanin protein CD9 and blue light illumination. Once the cargo proteins are introduced the exosomes via the process of endogenous biogenesis, they can be detached from CD9-conjugated CIBN by removal of the illumination source, resulting in their release into the intraluminal space of the exosomes and enabling efficient delivery to the cytosolic compartment of target cells.