Ex-Post Facto Law Law and Legal Definition

Ex post facto is a Latin term meaning “after the fact.” Ex-Post Facto Law is a law that retroactively changes the legal consequences of acts committed or the legal status of facts and relationships that existed prior to the enactment of the law. In U.S ex-post facto criminal laws are prohibited under the Constitution. Article I, section 10, clause 1 of the Constitution provides that no state shall pass any ex post facto law. Similarly Article I, section 9, clause 3 imposes the same prohibition upon the federal government. However retrospective civil laws may be allowed. Nonetheless under certain circumstances they may violate the Contract or Due Process Clauses of the Constitution. The prohibition on ex post facto laws operates solely as a restraint on legislative power. It has no application to changes in the law made by judicial decision.