Urbanization is increasing rapidly in tropical regions. Information on the response of tropical streams to urbanization is critically needed, but this information is limited. Here we summarize our current understanding of urban stream ecosystems in Puerto Rico, a tropical island with a high level of industrialization and large urban areas. We focused on 16 tributaries of the Rio Piedras watershed, which drains the San Juan metropolitan area, and 16 tributaries of the Turabo watershed, a rapidly developing suburban area. Urban effects on nutrients and aquatic macroinvertebrates were similar to those described for other geographic regions. PO₄, K, and Mg concentrations increased considerably with urbanization, PO₄-P ranged from <5 µg/L in streams draining forested watersheds to >500 µg/L in heavily urbanized streams. Macroinvertebrate assemblage composition changed to dominance by tolerant taxa (e.g., snails, Chironomidae) as the proportion of urban land increased in the subwatershed. In contrast, other factors did not follow expected responses to urbanization. Stream hydrology was equally flashy in streams draining forested and urbanized watersheds. Urban streams were commonly channelized, but in contrast to studies done elsewhere, channel incision was not greater in urban than in forested streams. Degree of urbanization was not related to the integrity of the fish assemblages. River connectivity seems to be more important than urbanization in determining fish assemblage composition in Puerto Rican streams because all native species are migratory (i.e., diadromous). Urbanization increased water temperature and microbial activity. Overall, urban streams in Puerto Rico presented some unexpected responses to urbanization that seem to be the result of its island or coastal characteristics. Examples from Puerto Rico provide information on how tropical streams might respond to the impacts of urbanization.