In this study, the viscosities of molten pure Sn, a Sn–Pb eutectic alloy, and Sn–Ag–Cu alloys, whose properties such as liquidus temperature were similar, were measured using an oscillating crucible viscometer over a temperature range of 483–668 K. For the Sn–Ag–Cu alloys, measurements were conducted on Sn–0.8Cu, Sn–1Ag–0.7Cu, and Sn–4Ag–0.5Cu alloys (mass%) with various Ag contents. The viscosities of molten pure Sn obtained using a viscometer ranged from 1.2 to 1.6 mPa·s, which were in good agreement with those reported in the literature. Additionally, the viscosity of the molten Sn–Pb alloy closely matched the values reported by T. Ejima, Y. Sato, T. Yamamura, A. Hayashi, and T. Yamazaki, “Viscosity of molten Sn–Pb alloys,” J. Jpn. Inst. Met., vol. 54, pp. 1005–1010, 1990, whereas that of the molten Sn–Ag–Cu alloys was similar to that reported by A. Adachi, Y. Ogino, and M. Shiraishi, “Viscosity of molten Sn–Ag–Cu alloys,” J. Jpn. Inst. Met., vol. 36, pp. 927–933, 1972. The molten Sn–Ag–Cu alloys also exhibited a temperature dependence similar to that of the pure molten Sn and Sn–Pb alloys. However, no significant changes in the viscosities were observed with varying Ag content. A temperature-dependent viscosity equation was developed for each alloy on the basis of the measured viscosity data.