Ultrathin two-dimensional (2D) nanomaterials composed of abundant and inexpensive 3d metal chalcogenides are competitive candidates for practical electrocatalysts for the oxygen evolution reaction (OER). However, the bottom-up synthesis of atomically thick nanosheets is difficult for materials with inherent non-layered host crystals. Here, we demonstrate the preparation of single-unit-cell thick Co9S8 nanosheets from preassembled Co14 nanoclusters (NCs) by virtue of the flexibility of NC self-assembly in colloidal solution. Due to their free-standing properties, the NC self-assembled architectures are capable of bearing sulfurization at elevated temperatures, thus producing ultrathin Co9S8 nanosheets. The nanosheets exhibit an OER overpotential as low as 0.27 V at 10 mA cm-2 in 0.1 M KOH, which is comparable to the performance of the best Co-based OER electrocatalysts.