Clostridia, named after the type genus Clostridium, are a group of bacteria common in most ecosystems, both terrestrial and marine.

They are generally Gram-positive bacteria and incapable of respiring oxygen, which is toxic to them. They feed on organic matter, which they ferment, producing reduced molecules such as alcohols or even hydrogen. Many of their species have the ability to form internally originated spores, extremely resistant to unfavorable conditions. They usually inhabit oxygen-poor sediments, decomposing organic matter, or in the interior of animals, although some species are parasitic or grow in oxygen-poor hypersaline environments. Certain species are important producers of diseases in humans, causing botulism or tetanus.