Bulk gold (Au) is known to be chemically inactive. However, when the size of Au nanoparticles (Au NPs) decreases to close to 1 nm or sub-nanometer dimensions, these ultrasmall Au nanoclusters (Au NCs) begin to possess interesting physical and chemical properties and likewise spawn different applications when working with bulk Au or even Au NPs. In this study, we found that it is possible to confer antimicrobial activity to Au NPs through precise control of their size down to NC dimension (typically less than 2 nm). Au NCs could kill both Gram-positive and Gram-negative bacteria. This wide-spectrum antimicrobial activity is attributed to the ultrasmall size of Au NCs, which would allow them to better interact with bacteria. The interaction between ultrasmall Au NCs and bacteria could induce a metabolic imbalance in bacterial cells after the internalization of Au NCs, leading to an increase of intracellular reactive oxygen species production that kills bacteria consequently.