Sea cucumbers are marine animals that belong to the class Holothuroidea, which is part of a larger animal group called echinoderms. They have an elongated body with a leathery skin and a single, branched gonad. Sea cucumbers are found on the sea floor worldwide, and they can be scavengers that feed on small food items in the benthic zone, as well as plankton floating in the water column. They use their tentacle-like tube feet for locomotion and feeding. Sea cucumbers can vary in size from less than an inch to over six feet, depending on the species.

Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter, after which bacteria can continue the decomposition process. They can confuse or harm predators by propelling their own toxic internal organs from their bodies in the direction of an attacker. Some species can also discharge sticky threads to ensnare their enemies, or violently contract their muscles and shoot some of their internal organs out of their rear ends as a defense mechanism.

Sea cucumbers are named for their resemblance to the fruit of the cucumber plant. They are found in virtually all marine environments throughout the world, from shallow to deep-sea environments. Sea cucumbers are enjoyed by humans, especially in Asia, and some species are farmed as delicacies.