Bacterial protein degradation machinery consists of chaperone–protease complexes that play vital roles in bacterial growth and development and have sparked interest as novel antimicrobial targets. ClpC–ClpP (ClpCP) is one such chaperone–protease complex, recruited by adaptors to specific functions in the model bacteriumBacillus subtilisand other Gram-positive bacteria, including the pathogensStaphylococcus aureusandMycobacterium tuberculosis. Here we have identified a new ClpCP adaptor protein, MdfA (metabolic differentiation factor A; formerly YjbA), in a genetic screen for factors that help driveB. subtilistoward metabolic dormancy during spore formation. A knockout ofmdfAstimulates gene expression in the developing spore, while aberrant expression ofmdfAduring vegetative growth is toxic. MdfA binds directly to ClpC to induce its oligomerization and ATPase activity, and this interaction is required for the in vivo effects ofmdfA. Finally, a cocrystal structure reveals that MdfA binds to the ClpC N-terminal domain at a location analogous to that on theM. tuberculosisClpC1 protein where bactericidal cyclic peptides bind. Altogether, our data and that of an accompanying study by Riley and colleagues support a model in which MdfA induces ClpCP-mediated degradation of metabolic enzymes in the developing spore, helping drive it toward metabolic dormancy.