We report that complexation-reaction-mediated two-dimensional crystalline assembly of gold (Au-14) nanoclusters (NCs) exhibits room-temperature delayed fluorescence at 605 nm, with an unprecedented long lifetime of 0.5 ms and an exceptionally high quantum yield of 19.1 +/- 0.9%. Interestingly, the as-synthesized Au NCs had a very weak delayed fluorescence signal. The enhancement in delayed fluorescence of Au NCs upon formation of assembly has been attributed to the crystallization induced structural rigidity, which restricted the nonradiative transitions and enhanced the excited-state lifetime. The attainment of crystalline organization was substantiated by electron diffraction analysis. A possible structure was established based on experimental results and computational optimizations. Atomic force microscopy revealed the formation of multilayered two-dimensional nanosheets with thickness of 2.44 +/- 0.48 nm.