The present invention addresses a problem of providing a grease composition for a speed reducer and a speed increaser, excellent in both leakage prevention performance and energy transfer efficiency. The grease composition for a speed reducer and a speed increaser contains a base oil (A), and nanofibers (B) having a thickness (d) of 1 to 500 nm, wherein the nanofibers (B) are one or more selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2).