Population movements, growth and industrialization are causing rapid urbanization throughout the tropics, resulting in elevated temperatures within urban areas when compared to surrounding rural areas, a phenomenon known as the Urban Heat Island (UHI). One such example is the city of San Juan, Puerto Rico. Our objective in this study is to quantify the UHI created by the San Juan Metropolitan Area over space and time. We collected temperature data using mobile and fixed-station measurements to quantify the UHI created by San Juan and its environs. We also used these data to examine the relation between average temperature at a given location and the density of vegetation located upwind. We then regressed regional temperatures against land-cover to predict future temperature with projected land-cover change. Our data show the existence of a nocturnal UHI, with nighttime urban-rural temperature differences (ΔTU-R) of up to 3.02°C. Urban-rural temperature differences had negligible seasonal differences. Comparisons of diurnal temperature trends at urban, grassland, and forested sites indicate that canopy cover reduced daytime warming. Results from the mobile measurements show that the UHI has reached the base of the Luquillo Mountains. Temperature was predicted best (r2 = 0.94) by vegetation in upwind southeasterly directions, especially that within 180 meters of the sensor. Predictions of future development and temperatures suggest that if the present pattern of development continues, over 140 km2 of land that showed no signs of UHI in 2000 will have an average annual UHI between +0.4°C and +1.55°C by 2050. Furthermore, more than 130 km2 of land area with a current UHI between +0.4°C and +1.4°C in 2000 will have an average UHI greater than +1.55°C by 2050.