Nematodes play an important role in ecological processes and are one of the most abundant meiofaunal organisms associated with seaweeds. Yet, knowledge on seaweed bed ecosystems is limited. Nematodes associated with Sargassum polyceratium and Halimeda opuntia were compared in two transects, 80 m apart and parallel to the beach line in Cupe Beach, Brazil. The temporal variation during the dry and rainy seasons and the effect of sediment retention by the seaweed on nematode density and composition were investigated. The differences in nematode communities between the two seasons were mainly caused by the increase in density of the most abundant genera in the rainy season. A significant difference was observed between the nematode communities of the two transects for H. opuntia. The nematode communities of both seaweed species did not differ significantly in the same transect. The genus Euchromadora was dominant in both seaweed species. The amount of sediment retained by the seaweeds did not affect the overall nematode density. However, it was positively correlated with the density of Draconema and Euchromadora in both seaweeds, and both genera were exclusively found associated with seaweeds. This result opposes the idea that the more sediment retained by the seaweed, the higher the nematode overall density and the higher the number of nematodes originally coming from the sediment.