Evapotranspiration estimates in forested areas are important not only for water resource management on a regional scale but also to better understand the water cycle on a global scale. The objective of this study was to evaluate the Global Change Observation Mission-Climate (GCOM-C) global Evapotranspiration-index (ET_index) estimation algorithm (GCOM-C ET_index algorithm) applied to forested areas. ET_index, which is the ratio of the actual evapotranspiration to the reference evapotranspiration, is estimated from the actual surface temperature and hypothetical wet and dry surface temperatures, i.e., T_s (wet) and T_s (dry), respectively. Based on the algorithm, evapotranspiration is calculated from thermal satellite images and near-surface weather data. We compared the observed ground-based annual evapotranspiration with the estimated annual evapotranspiration obtained using the GCOM-C ET_index algorithm and thermal images from the Moderate Resolution Imaging Sectroradiometer (MODIS) satellite for 40 forests, with 10 sites in four different areas, including Japan, North America, Australia, and the tropical region. We found that the GCOM-C ET_index algorithm well reproduced annual evapotranspiration for most forests. The root mean square errors (RMSE) for the 40 forests was 239 mm. In Japan, North America, and Australia, the overestimation of summer evapotranspiration was offset by the underestimation of winter evapotranspiration. The accuracy of annual evapotranspiration estimates in forests with low annual mean temperatures (<15 °C) was less than that in forests with high annual mean temperatures (≥15 °C). Forests with a low annual mean temperature displayed low levels of evapotranspiration in winter. In these forests, the overestimation of summer evapotranspiration was not offset by the underestimation of winter evapotranspiration. The overestimation of T_s (wet) is the primary reason for the overestimation of summer evapotranspiration. Redetermination of the parameters for the T_s (wet) estimates must improve the evapotranspiration estimates in the forested areas, especially the ones with a low annual mean temperature.