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The mechanisms of gold bioaccumulation by cyanobacteria (Plectonema boryanum UTEX 485) from gold(III)−chloride solutions have been studied at three gold concentrations (0.8, 1.7, and 7.6 mM) at 25 °C, using both fixed-time laboratory and real-time synchrotron radiation absorption spectroscopy (XAS) experiments. Interaction of cyanobacteria with aqueous gold(III)−chloride initially promoted the precipitation of nanoparticles of amorphous gold(I)−sulfide at the cell walls, and finally deposited metallic gold in the form of octahedral (111) platelets (∼10 nm to 6 μm) near cell surfaces and in solutions. The XAS results confirm that the reduction mechanism of gold(III)−chloride to metallic gold by cyanobacteria involves the formation of an intermediate Au(I) species, gold(I)−sulfide.