宣誓下作假证供（英语：Perjury，又称发假誓、伪证罪、作伪证），指在正式宣誓后说谎或作出被证明为不实的陈述。它是一种刑事罪行，因为证人宣誓过会说真话，而为了确保法庭的公信力，证人的证供必须可信。宣誓下作假证供会剥夺法庭公平审判的能力，造成误判，所以是严重的罪行。

就事实提出分析论述并不构成此罪，因人受推理能力所限，经常有不精确的结论，这并非故意的。人可以十分诚实，但抱着错误的观念，他们的记忆力也可能含糊。和其他普通法罪行一样，罪犯除了要有犯法的意图（mens rea）外，还必须有犯罪行为（actus reus）。

Perjury, also known as forswearing, is the intentional act of swearing a false oath or of falsifying an affirmation to tell the truth, whether spoken or in writing, concerning matters material to an official proceeding. Contrary to popular misconception, no crime has occurred when a false statement is (intentionally or unintentionally) made while under oath or subject to penalty—instead, criminal culpability only attaches at the instant the declarant falsely asserts the truth of statements (made or to be made) which are material to the outcome of the proceeding. For example, it is not perjury to lie about one's age except where age is a fact material to influencing the legal result, such as eligibility for old age retirement benefits or whether a person was of an age to have legal capacity.