The present study describes the variation in the benthic macrofauna related to the presence of Sabellaria wilsoni (Polychaeta: Sabellariidae) reefs on a sandy beach of the Brazilian Amazon Coast. The study also investigated whether the erosion of parts of the reef by intense wave action results in differences in the fauna. Samples were collected from a patch of reef and the adjacent sandy sediment for the analysis of the zoobenthos and substrates (granulometry and organic matter content) on Algodoal-Maiandeua Island (northern Brazil coast). The reef had more heterogeneous sediments and a higher organic matter content, and its fauna was distinct from that of the beach, with a higher density, species richness and diversity. The reef fauna included taxa typical of both consolidated and unconsolidated substrates. The portion of the reef more exposed to wave action had a lower density of reef-building worms, and these worms were smaller in size than those of the more protected portion, although the associated fauna of the exposed portion was denser and richer in species. These results confirmed that S. wilsoni is an important ecosystem engineer on the Amazon coast, and that the diversity and unique features of the fauna associated with these reefs emphasize their importance as a substrate for the local benthic communities, in particular in areas where consolidated bottoms are naturally scarce.