Tropical thin cirrus clouds play an important role in the earth's climate system through their longwave radiative forcing and effect on water vapor distribution. With a unique band centered at 1.375 µm, the Moderate Resolution Imaging Specroradiometer (MODIS) instruments on both the TERRA and AQUA satellite platforms provide an unprecedented opportunity to study thin cirrus clouds. The optical thickness of thin cirrus clouds is one of the most important parameters for the understanding of the forcing of these clouds in the earth-atmosphere energy budget. In this paper, we introduce a method to retrieve the optical thickness of tropical cirrus clouds using MODIS visible (0.66 µm) and 1.375-µm bands in conjunction with forward scattering and radiative transfer modeling simulations. We demonstrate the applicability of this algorithm using several independent MODIS images from NASA’s Terra satellite. The method described here is complimentary to the MODIS operational cloud retrieval algorithm for the case of cirrus clouds.