Urbanization has many negative impacts on stream ecosystems. Among them, increases in water temperature are important as can influence rates of ecosystem processes. We studied the degree of water warming created by urban areas in Puerto Rico and its effects on microbial respiration rates. We expected to find warmer water temperature in urban than in forested watersheds, and a positive relation between water temperature and microbial respiration on leaf litter. Using USGS data, we found that urban streams in the Rio Piedras watershed, located in the San Juan metropolitan area, had water temperature 2-3 degrees warmer than similar streams in non-urban watersheds. Water temperature in urban streams ranged from 24-29 °C while non-urban ranged from 22-25 °C. Microbial respiration response to temperature was assessed in the laboratory, incubating leaves and measuring dissolved oxygen at different temperatures ranging from 23-38 °C. Microbial respiration increased with water temperature until a threshold is reached at about 32 °C. Overall, our study shows that microbial respiration is strongly influenced by water temperature and that urbanization has a strong potential to accelerate ecosystem processes in streams.