Bad faith (Latin: mala fides) is a sustained form of deception which consists of entertaining or pretending to entertain one set of feelings while acting as if influenced by another. It is associated with hypocrisy, breach of contract, affectation, and lip service. It may involve intentional deceit of others, or self-deception.

Some examples of bad faith include: soldiers waving a white flag and then firing when their enemy approaches to take prisoners (cf. perfidy); a company representative who negotiates with union workers while having no intent of compromising; a prosecutor who argues a legal position that he knows to be false; and an insurer who uses language and reasoning which are deliberately misleading in order to deny a claim.

In philosophy, after Jean-Paul Sartre's analysis of the concepts of self-deception and bad faith, the latter concept has been examined in specialized fields as it pertains to self-deception as two semi-independently acting minds within one mind, with one deceiving the other. Bad faith may be viewed in some cases to not involve deception, as in some kinds of hypochondria with actual physical manifestations. There is a question about the truth or falsity of statements made in bad faith self-deception; for example, if a hypochondriac makes a complaint about their psychosomatic condition, is it true or false?

Bad faith has been used as a term of art in diverse areas involving feminism, racial supremacism, political negotiation, insurance claims processing, intentionality, ethics, existentialism, climate change denial, and the law.