Nanodiamonds (NDs) are a new and attractive class of materials for sensing and delivery in biological systems. Methods for functionalizing ND surfaces are highly valuable in these applications, yet reported approaches for covalent modification with biological macromolecules are still limited, and characterizing behaviors of ND-tethered biomolecules is difficult. Here we demonstrated the use of copper-free click chemistry to covalently attach DNA strands at ND surfaces. Using site-directed spin labeling and electron paramagnetic resonance spectroscopy, we demonstrated that the tethered DNA strands maintain the ability to undergo repetitive hybridizations and behave similarly to those in solutions, maintaining a large degree of mobility with respect to the ND. The work established a method to prepare and characterize an easily addressable identity tag for NDs. This will open up future applications such as targeted ND delivery and developing sensors for investigating biomolecules.