Fluorescent nanodiamond (FND) is attracting much attention as a bioimaging agent because of its inherent biocompatibility and superior optical properties (e.g., excellent photostability and far-red emission). However, for practical use in life science research, some issues such as higher brightness and ease of bioconjugation have to be solved. Here, it is shown that the 100-nm FND particles fabricated by using nitrogen-rich type Ib diamonds and high-energy proton irradiation are highly fluorescent and readily functionalizable with proteins for biological applications. In the first approach, acid-treated FND is noncovalently coated with glycoproteins or neoglycoproteins (i.e., proteins chemically modified with multiple sugar residues) for targeting hepatocytes via carbohydrate receptors. In the second approach, FND is first PEGylated and then covalently conjugated with streptavidin, to which biotin-labeled antibodies of interest are linked. High targeting specificity of the bioconjugated FND is demonstrated with the human hepatoma cell line, HepG2, and breast cancer cell lines, ASB145-1R, MCF-7, and MDA-MB-231 cells. These approaches should be widely applicable to a variety of situations for specific targeting and labeling of cells.