The optical properties of metal nano particles have attracted wide interest. Recent progress in controlling nanoparticles with atomic precision (often called nanoclusters) provide new opportunities for investigating many fundamental questions, such as the transition from excitonic to plasmonic state, which is a central question in metal nanoparticle research because it provides insights into the origin of surface plasmon resonance (SPR) as well as the formation of metallic bond. However, this question still remains elusive because of the extreme difficulty in preparing atomically precise nano particles larger than 2 nm. Here we report the synthesis and optical properties of an atomically precise Au-279 (SR)(84) nanocluster. Femtosecond transient absorption spectro-scopic analysis reveals that the Au-279 nanocluster shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au-246(SR)(80) nanocluster. Steady-state absorption spectra reveal that the nascent plasmon band of Au-279 at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. The sharp transition from nonmetallic Au-246 to metallic Au-279 is surprising and will stimulate future theoretical work on the transition and many other relevant issues.