Droplets are ideally suited to support high‐Q‐factor whispering gallery modes (WGMs) due to their perfectly smooth surface. WGMs enable extremely precise measurements of the droplet properties such as size and shape. Herein, a simple, fast, and very precise technique to measure interfacial tension (IFT) between two immiscible liquids based on WGMs is demonstrated. A microdroplet is generated at the end of a glass microcapillary, submerged in a continuous liquid phase, and its size changes are monitored with nanometer precision via WGMs, while simultaneously applying finely tunable pressure through the microcapillary. IFT is determined from the size of the droplet and the pressure in the microcapillary at equilibrium. Droplets as small as 8 μm are used, thus requiring extremely small sample volume. The IFT measurements can be carried out also at nonequilibrium state when either the size of the droplet or the chemical composition of the continuous phase changes in time. Simultaneously, the WGMs enable very precise measurement of the refractive index of either the droplet or the continuous phase.