Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal X-ray structures has been instrumental to enable advanced molecular understanding of their intriguing properties. So far, however, a general, reliable, chemically clean approach to prepare single crystals suitable for accurate crystallographic analysis was missing. Here we show that single crystals-of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystallization. This method relies on the fact that charged clusters, display a higher solubility in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insoluble clusters directly onto the electrode surface eventually leads to the formation of a dense forest of millimeter-long single crystals. Electrocrystallization of three known Au-25(SR)(18)(0) clusters is described. A new cluster; Au-25(S-nC(5)H(11))(18), was also prepared and found to crystallize by forming bundles of millimeter-slong Au-25 polymers.