To provide a surface-modified nanodiamond including an N-substituted or unsubstituted amino group having excellent reactivity with other compounds; and having excellent dispersibility in dispersion media and resins. The surface-modified nanodiamond according to the present invention is a surface-modified nanodiamond including a surface-modifying group on a surface of a nanodiamond particle, wherein the surface-modifying group includes a group represented by Formula (1) below, wherein reference signs in Formula (1) below are as described in the present specification.