Silver nanoclusters have attracted scientific interest due to their properties and applications. However, practical synthetic methods to access these materials are still limited mainly due to the low stability. Here, we report a controlled assembly strategy for fabricating atomically precise silver nanoclusters using polyoxometalates (POMs) as structure-directing as well as functionalizing units. A trefoil-propeller-shaped {Ag-27}(17+) nanocluster was synthesized by assembling reactive nano clusters supported by open-Dawson-type POMs [si(2)w(18)O(66)](16-). The {Ag-27}(17+) nanocluster possessed 10 delocalized valence electrons and showed unprecedented ultrastability in solutions. The cluster showed unique {Ag-27}-to-POM charge transfer bands in the visible light region.