Irrigated agriculture in the Texas High Plains accounts for a major portion of the groundwater withdrawals from the Ogallala aquifer, and groundwater levels are declining. Accurate information on irrigated acreage and its spatial distribution enhances local groundwater districts’ ability to manage limited water resources. In addition, irrigated land area is one of the important inputs in most surface and groundwater models to evaluate economic feasibility for various crop rotations systems and irrigation management practices. In this study, we used a novel approach to derive an irrigated area map covering a 4-county area (Dallam, Sherman, Hartley, and Moore Counties) in the northwest region of the Texas High Plains from a Landsat 5 Thematic Mapper data image acquired on August 13, 2008. The spectral band ratios and vegetation indices were used to define threshold value for the irrigated pixels. The hierarchical rule-based decision tree classification algorithm was employed to delineate final irrigated class. Ground truth data collected for accuracy assessment included land cover type, irrigation practices and their geographic locations using a global positioning system. Accuracy assessment of the irrigated area map indicated that we achieved an overall mapping accuracy of 96% with omission and commission errors at 9% and 8%, respectively, which are mainly due to clouds and shadows of clouds. Irrigated acreages derived from the TM image closely matched with that from agricultural statistical reports for the 4-county area. This map will be used in the comprehensive regional analysis of groundwater depletion in the Ogallala Aquifer Region with the purpose of understanding short- and long-term effects of existing and alternative land use scenarios on groundwater changes.