Urbanization is a major land use form that has large impacts on ecosystems. Urban development in the watershed impacts stream ecosystems by increasing nutrient and organic matter loads, altering hydrology, and reducing biodiversity. Puerto Rico is an ideal location to assess and monitor the effects of urbanization on streams, because it is increasingly urbanized and streams do not receive inputs of untreated sewage, characteristic of many other tropical urban areas. The objective of this study was to determine how leaf-litter decomposition and aquatic macroinvertebrate assemblages varied along a tropical urban gradient. We conducted the study in the Río Piedras watershed, San Juan Metropolitan Area, in six low order streams that formed an urban gradient ranging from 10 to 70% urban land cover. At each stream, we placed six 5g leaf bags of Ficus longifolia in three different pools and collected one bag on each sampling date. Decomposition rates were fast in forested streams (range 0.021 – 0.039 day-1) and decreased with increasing urbanization (range 0.007 – 0.008 day-1). Rates were strongly and negatively correlated with percent impervious surface cover (R= 0.81, p=0.01). Functional feeding group diversity was higher in forested streams, with the presence of shredders. Decomposition rates were significantly and positively correlated with functional feeding group diversity and abundance (R= 0.66, p= 0.04). Overall, our results show that urbanization affected the environment and macroinvertebrate diversity resulting in large negative effects on stream ecosystem function.