Thermal infrared satellite sensor images are widely used for assessing the thermal urban environment as well as for identifying the Urban Heat Islands (UHI) in highly urbanized areas. In this study, the relationship between urban growth and UHI in the megacity of Istanbul was investigated using the Landsat 5 TM satellite sensor data and temperature recordings obtained from the meteorological stations within and around the city of Istanbul. Using the thermal band of the Landsat 5 TM images, the spatial distribution of the warmer surfaces in the urban environment are identified and matched with the urban surface characteristics together with the land use/cover distribution. Three Landsat 5 TM satellite sensor imageries obtained 25 September 1987, 18 July 1997, and 28 June 2007 were used. UHI boundaries were extracted from the classified thermal band of these satellite sensor images. Images were also classified to form the link between radiances, surface temperatures, and land use characteristics particularly for the urbanized areas. The results of the study indicated the expansion of urban sprawl in the city that further corresponded to growth in thermal radiation of land surface in high-density areas. This fact is mainly due to unplanned and uncontrolled urbanization that has occurred in the last three decades.