Adversary may refer to:

• In general, one's adversary is someone whose purposes are opposed to, or conflict with, one's own purposes
• Adversary (cryptography), in cryptography, a malicious entity whose aim is to prevent the users of the cryptosystem from achieving their goal
• In computer algorithms, an adversary is a theoretical agent that uses information about the past moves of an on-line algorithm to choose inputs that force the worst-case cost of the algorithm.
• In religious contexts, Adversary may refer to the Devil.
• The Adversarial process is used to resolve disputes
• The Adversarial system is used in legal processes
• The Adversary proceeding is used in bankruptcy processes
• The Adversarial review is used as a part of the scientific process

Relating to popular culture:

• Adversary (band), a metal band from Århus, Denmark
• Adversary (band), a metal band from Chicago, USA
• Ad·ver·sary, an industrial band from Ottawa, Canada
• The Adversary is the main villain in Fables
• Adversary (comics), a villain from the Marvel comics universe
• The Adversary (DS9 episode), the season finale of Star Trek: Deep Space Nine's third season