Urbanization is degrading stream ecosystems worldwide. Tropical island streams may respond to urbanization differently than temperate streams because of their overall climate differences, and they may respond differently than continental tropical urban streams because of their reduced biological diversity and short drainages. We characterized the physicochemistry, physical habitat, and macroinvertebrate assemblages of 16 stream tributaries in the Rio Piedras Watershed (San Juan, Puerto Rico). We also described landuse patterns upstream from each sampling site for the entire subwatershed and for riparian buffers of 5- and 100-m width. Urbanization had a negative effect on the physicochemical and biological condition of the Rio Piedras. Streams were distributed in ordination space along a strong physicochemical gradient that was related to concentrations of K , Mg² , dissolved O₂ (DO), and PO₄³⁻. Along this gradient, DO and Mg² decreased and PO₄³⁻ and K increased with higher % urban cover in the subwatershed. Macroinvertebrate assemblages also were related to urbanization, and more macroinvertebrate families and pollution-sensitive taxa were found at sites where physicochemistry reflected less urban cover. Family richness and pollution-sensitive taxa were positively associated with greater % forest cover in the 5-m riparian buffer zone, a result that supports the use of riparian buffers to ameliorate the effects of urbanization on stream biointegrity in the Rio Piedras. Our results are similar to findings in urban streams in temperate zones and in tropical continental streams. Therefore, despite island characteristics, tropical island stream physicochemistry and macroinvertebrate assemblages responded to urbanization in ways that are in general agreement with the predictions of the Urban Stream Syndrome.