Discrete gold nanoparticles with diameters between 2 and 3 nm show remarkable properties including enhanced catalytic behavior and photoluminescence. However, tunability of these properties is limited by the tight size range within which they are observed. Here, we report the synthesis of discrete, bimetallic gold-copper nanoparticle alloys (diameter congruent to 2-3 nm) which display photoluminescent properties that can be tuned by changing the alloy composition. Electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and pulsed-field gradient stimulated echo H-1 NMR measurements show that the nanoparticles are homogeneous, discrete, and crystalline. Upon varying the composition of the nanoparticles from 0% to 100% molar ratio copper, the photoluminescence maxima shift from 947 to 1067 nm, with excitation at 360 nm. The resulting particles exhibit brightness values (molar extinction coefficient (epsilon) X quantum yield (Phi)) that are more than an order of magnitude larger than the brightest near-infrared-emitting lanthanide complexes and small-molecule probes evaluated under similar conditions.