A resin composition (X), which is a curable resin composition according to the present invention, contains a curable resin component (10), a surface-modified nanodiamond (20), and an organic medium. The surface-modified nanodiamond (20) includes a ND particle (21), which is a nanodiamond particle, and a silane coupling agent (22). The silane coupling agent (22) has a (meth)acryloyl-containing organic chain and is bonded to the ND particle (21). The resin composition (X) is suitable for forming a nanodiamond-dispersed cured resin portion having both high transparency and high scratch resistance. An optical element according to the present invention has a light-transmitting region and includes a cured resin portion, which is a cured product of the resin composition (X), in at least part of the light-transmitting region. The optical element is advantageous for actually providing an optical element including a cured resin portion or cured resin layer having both high transparency and high scratch resistance.