Single-nanoparticle-level sensing allows us to measure individual molecular interactions and probe environmental stimuli at nanometer-scale resolution. Despite these premises, limited success has been met hitherto due to the demanding challenge to distinguish a dimmed signal from a noisy background. Here, we describe an approach for high-sensitivity single-nanoparticle-level sensing of divalent copper (Cu2+) ions through near-infrared-to-visible upconversion luminescence against a near-null background. This nanosensor utilizes ytterbium- (Yb3+) and erbium (Er3+)-doped sodium yttrium fluoride (NaYF4) upconversion nanoparticles (UCNPs) (maximal emission at 540 nm when excited at 980 nm) as an energy donor, of which the surface attaches Cu2+-dependent DNAzymes labeled with BHQ1 dye (Black Hole Quencher 1, maximal absorption at 548 nm) as energy acceptors. Adding a hint amount of Cu2+ ions resulted in the cleavage of a BHQ1-containing moiety in DNAzymes, thus turning on upconversion luminescence for sensitive detection. Indeed, this approach allows us to perform single-nanoparticle-level detection of Cu2+ ions with extraordinary signal-to-noise ratios (SNRs, >277) for all measured concentrations that cover 3 orders of magnitude (from sub-nM to μM). Importantly, a limit of detection of 220 pM was achieved, about sevenfold lower than the one at the ensemble level. Moreover, a stochastic particle-to-particle sensing behavior was also identified, featuring single-nanoparticle-level detection. This work untaps the usage of UCNPs for high-sensitivity single-nanoparticle-level biosensing.