Several studies have demonstrated that areas of seagrass show significantly greater density, richness, and diversity of fauna than adjacent unvegetated areas. Most investigations have dealt with midsized and larger seagrass species that usually form dense, highly structured meadows. Similar studies of smaller seagrasses have rarely been carried out. The objective of this study was to investigate whether the small, low biomass Halophila decipiens Ostenfeld confirms the paradigm observed in larger seagrass species by showing a greater macrofaunal density and diversity than adjacent bare, soft-bottom areas. Plant mean biomass at Urca inlet was 1.83 g m⁻². Crustacea were the most abundant, frequent, and constant group in both habitats. Six taxa were significantly more abundant in the vegetated habitat, while only one taxon was enhanced in the habitat without H. decipiens. Total mean density, richness, and diversity of macrofauna were significantly higher in vegetated areas. Habitat complexity, higher productivity, or both associated with the seagrass were responsible for the differences found. Our results confirm and reinforce the paradigm that seagrass habitats have a greater density and diversity of macrofauna than adjacent unvegetated areas. However, adjacent unvegetated areas can present different sets of species that, even though less diverse, contribute to enhance local diversity.