AbstractAg clusters with a controlled number of atoms have received significant interest because they show size‐dependent catalytic, optical, electronic, or magnetic properties. However, the synthesis of size‐controlled, ligand‐free, and air‐stable Ag clusters with high yields has not been well‐established. Herein, it is shown that isostructural porous ionic crystals (PICs) with redox‐active polyoxometalates (POMs) can be used to synthesize Ag clusters via electron transfer from POMs to Ag⁺. Ag clusters with average numbers of three, four, or six atoms emitting blue, green, or red colors, respectively, are formed and stabilized in the PICs under ambient conditions without any protecting ligands. The cluster size solely correlates with the degree of electron transfer, which is controlled by the reduction time and types of ions or elements of the PICs. Thus, advantages have been taken of POMs as electron sources and PICs as scaffolds to demonstrate a convenient method to obtain few‐atom Ag clusters.