Several species of bivalves belonging to families which are typically associated with reducing conditions, like those observed at methane seep sites, have been obtained in recent explorations of the Caribbean Sea margin off the coast of Colombia. The material has been collected at depths of around 500 m west of the Magdalena and Sinú deltas, located in the Sinú–San Jacinto fold belt. These bivalves correspond to the families Vesicomyidae (Calyptogena ponderosa, Vesicomya caribbea and Ectenagena modioliforma), Lucinidae (Graecina colombiensis and three unidentified species of Lucinoma), Solemyidae (Acharax caribbaea) and Thyasiridae (Conchocele bisecta). In addition, for the first time off Colombia empty tubes of vestimentiferan polychaetes, belonging to the family Siboglinidae, were collected. At some of these sites the presence of authigenic carbonates has been observed together with the biological material. Although the obligate seep fauna generally contains relatively few and endemic species, a large suite of accompanying heterotrophic species (here we report only the molluscs) has been found at the seep sites. The occurrence of carbonates, the geological characteristics of the area and the new biological evidence confirms the presence of methane seep ecosystems in the Caribbean Sea off Colombia.