Hydra may stand for:

• a simple fresh-water animal of the class Hydrozoa. See Hydra (biology).
• a constellation. See Hydra (constellation). Sometimes Hydra is confused with the constellation Hydrus.
• in Greek mythology, a many-headed serpent which Heracles defeated. See Lernaean Hydra.
• one of the Saronic Islands in Greece. See Hydra, Saronic Islands.
• the name of a distributed operating system developed at Carnegie Mellon University.
• in the Marvel Universe, HYDRA is a secret criminal organization (originally affiliated with the Nazis) fighting for world domination. See HYDRA (comics).
• a computer chess program (based on software and FPGA hardware).