The present invention is to provide a surface-modified nanodiamond having excellent affinity for an organic solvent and a resin and having high dispersibility in an organic solvent and in a resin. The surface-modified nanodiamond according to an embodiment of the present invention has a structure in which a surface of a nanodiamond particle is modified by a group represented by Formula (1) below. In Formula (1), R1 represents an aliphatic hydrocarbon group having 6 or more carbons. R2 and R3 may be the same or different and are each a hydrogen atom, an aliphatic hydrocarbon group having from 1 to 3 carbons, or a group represented by Formula (2) below.