Riftia pachyptila, commonly known as the giant tube worm and less commonly known as the giant beardworm, is a marine invertebrate in the phylum Annelida (formerly grouped in phylum Pogonophora and Vestimentifera) related to tube worms commonly found in the intertidal and pelagic zones. R. pachyptila lives on the floor of the Pacific Ocean near hydrothermal vents. The vents provide a natural ambient temperature in their environment ranging from 2 to 30 °C, and this organism can tolerate extremely high hydrogen sulfide levels. These worms can reach a length of , and their tubular bodies have a diameter of .
Its common name "giant tube worm" is, however, also applied to the largest living species of shipworm, Kuphus polythalamius, which despite the name "worm", is a bivalve mollusc rather than an annelid.
R. pachyptila was discovered in 1977 on an expedition of the American bathyscaphe DSV Alvin to the Galápagos Rift led by geologist Jack Corliss. The discovery was unexpected, as the team was studying hydrothermal vents and no biologists were included in the expedition. Many of the species found living near hydrothermal vents during this expedition had never been seen before.
At the time, the presence of thermal springs near the midoceanic ridges was known. Further research uncovered aquatic life in the area, despite the high temperature (around 350–380 °C).
Many samples were collected, including bivalves, polychaetes, large crabs, and R. pachyptila. It was the first time that species was observed.
R. pachyptila develops from a free-swimming, pelagic, nonsymbiotic trochophore larva, which enters juvenile (metatrochophore) development, becoming sessile, and subsequently acquiring symbiotic bacteria. The symbiotic bacteria, on which adult worms depend for sustenance, are not present in the gametes, but are acquired from the environment through the skin in a process akin to an infection. The digestive tract transiently connects from a mouth at the tip of the ventral medial process to a foregut, midgut, hindgut, and anus and was previously thought to have been the method by which the bacteria are introduced into adults.