Upconverting nanoparticles (UCNPs) are a highly attractive tool owing to their unique property of showing visible luminescence when excited in the near-infrared (NIR) region. Plain UCNPs have no biorecognition capabilities, but functionalization of their surface with azido groups renders them conjugatable to ethynyl-modified oligonucleotides in a bioorthogonal fashion. Single-stranded DNA was covalently attached to the surface of UCNPs by click chemistry and purified by size exclusion chromatography (SEC) at elevated temperature. Covalent attachment was evidenced by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. DNA conjugation makes the particle soluble in water and enables it to recognize its counter strand. Such UCNPs are capable of nonspecifically crossing cell membranes. Confocal microscopy reveals the high potential of the bright UCNPs for live cell imaging in the NIR, where the UCNP–DNA conjugates can be considered to act as a kind of nano-sized lamp. Furthermore, cross-linking of those DNA nanolamps yields highly emissive aggregates.