制宪议会（constitutional convention、constituent assembly或constitutional assembly）是指一个为起草和通过一部宪法而成立的一个主体。作为一个立国的根本性条文，一部宪法不能用简单地通过一般的国家立法机构修改或修订；而需要通过创建制宪议会制定宪法内容。一个制宪议会通常只是为某个特定的制宪目的而成立，在一段特定的较短的时期内召开，任务完成后便会解散。

不同于其它由地区统治者单方面制定宪法的制宪方式，制宪议会编订宪法采用“国内施行”机制，制宪议会成员都是宪法所在地的公民但不必一定是该地的统治者。正如 Jon Elster，哥伦比亚大学社会科学教授，所说：

A constituent assembly (sometimes also known as a constitutional convention or constitutional assembly) is a body or assembly of representatives composed for the purpose of drafting or adopting a constitution. As the fundamental document constituting a state, a constitution cannot normally be modified or amended by the state's normal legislative procedures; instead a constituent assembly, the rules for which are normally laid down in the constitution, must be set up. A constituent assembly is usually set up for its specific purpose, which it carries out in a relatively short time, after which the assembly is dissolved. A constituent assembly is a form of representative democracy.