Singlet oxygen (1O2) plays an important role in various applications, such as in the photodynamic therapy (PDT) of cancers, photodynamic inactivation of microorganisms, photo-degradation of toxic compounds, and photo-oxidation in synthetic chemistry. Recently, water-soluble metal nanoclusters (NCs) have been utilized as photosensitizers for the generation of highly reactive 1O2 because of their high water solubility, low toxicity, and surface functionalizability for targeted substances. In the case of metal NC-based photosensitizers, the photo-physical properties depend on the core size of the NCs and the core/ligand interfacial structures. A wide range of atomically precise gold NCs have been reported; however, reports on the synthesis of atomically precise silver NCs are limited due to the high reactivity and low photostability (i.e., easy oxidation) of Ag NCs. In addition, there have been few reports on what kinds of metal NCs can generate large amounts of 1O2. In this study, we developed a new one-pot synthesis method of water-soluble Ag7(MBISA)6 (MBISA = 2-mercapto-5-benzimidazolesulfonic acid sodium salt) NCs with highly efficient 1O2 generation ability under the irradiation of white light emitting diodes (LEDs). The molecular formula and purity were determined by electrospray ionization mass spectrometry and gel electrophoresis. To the best of our knowledge, this is the first report on atomically precise thiolate silver clusters (Agn(SR)m) for efficient 1O2 generation under visible light irradiation. The 1O2 generation efficiency of Ag7(MBISA)6 NCs was higher than those of the following known water-soluble metal NCs: bovine serum albumin (BSA)-Au25 NCs, BSA-Ag8 NCs, BSA-Ag14 NCs, Ag25(dihydrolipoic acid)14 NCs, Ag35(glutathione)18 NCs, and Ag75(glutathione)40 NCs. The metal NCs examined in this study showed the following order of 1O2 generation efficiency under white light irradiation: Ag7(MBISA)6 > BSA-Ag14 > Ag75(SG)40 > Ag35(SG)18 > BSA-Au25 ≫ BSA-Ag8 (not detected) and Ag25(DHLA)14 (not detected). For further improving the 1O2 generation of Ag7(MBISA)6 NCs, we developed a novel fluorescence resonance energy transfer (FRET) system by conjugating Ag7(MBISA)6 NCs with quinacrine (QC) (molar ratio of Ag NCs to QC is 1 : 0.5). We observed the FRET process, from QC to Ag7(MBISA)6 NCs, occurring in the conjugate. That is, the QC works as a donor chromophore, while the Ag NCs work as an acceptor chromophore in the FRET process. The FRET-mediated process caused a 2.3-fold increase in 1O2 generation compared to that obtained with Ag7(MBISA)6 NCs alone. This study establishes a general and simple strategy for improving the PDT activity of metal NC-based photosensitizers.