We report a contactless surface tension measurement method of micrometer-sized aerosol droplets. In this method, we assume spherical spontaneous resonance of a thermally induced capillary wave. First, an aerosol droplet with a radius ranging from 4.7 to 12.4 μm is trapped by means of a simple single-beam optical trapping configuration, and the frequency shift power spectrum of the light passing the droplet is measured. The spectrum in each case exhibits several peaks in a frequency range of several tens to several hundred kilohertz. The peak frequencies agree well with theoretical ones predicted by the spherical resonant modes. After validating the above-mentioned assumption, we measure the surface tension of aerosol droplets containing sodium dodecyl sulfate, and we successfully obtain the surface tension value. The present method utilizes just two phenomena, that is, the droplet surface light scattering and spontaneous resonance of the capillary wave. These can be easily observed in aerosol droplets, and they can be utilized to gain scientific insights. The present method based on the nature of droplets can be used in various applications in aerosol science.