Thin cirrus are often difficult to detect, and their optical properties difficult to retrieve, using passive sensors such as the Moderate-resolution Imaging Spectroradiometer (MODIS). Indeed, the MODIS cloud products (MOD06) often fail to retrieve such clouds. To potentially improve upon the capabilities of these products, a new optical thickness retrieval method, employing the MODIS 1.375 µm channel, has been developed for thin cirrus conditions. The 1.375 µm channel, centered within a strong water vapor absorption band, is quite sensitive to cirrus, or more specifically, to cirrus optical thickness. However, there are limitations that must first be overcome. For instance, the 1.375 µm signal suffers non-negligible attenuation due to atmospheric water vapor above and within the cloud. This attenuation can be accounted for by pairing 1.375 µm with a window channel, such as 1.24 µm. After correcting for attenuation, cirrus optical thickness is derived using a look-up table approach. The applicability of this method is demonstrated with retrieval results for several case studies.