A constituent assembly (also known as a constitutional convention, constitutional congress, or constitutional assembly) is a body assembled for the purpose of drafting or revising a constitution. Members of a constituent assembly may be elected by popular vote, drawn by sortition, appointed, or some combination of these methods. Assemblies are typically considered distinct from a regular legislature, although members of the legislature may compose a significant number or all of its members. As the fundamental document constituting a state, a constitution cannot normally be modified or amended by the state's normal legislative procedures in some jurisdictions; instead a constitutional convention or 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.

Unlike forms of constitution-making in which a constitution is unilaterally imposed by a sovereign lawmaker, the constituent assembly creates a constitution through "internally imposed" actions, in that members of the constituent assembly are themselves citizens, but not necessarily the political leaders, of the country for which they are creating a constitution. As described by Columbia University Social Sciences Professor Jon Elster:

"Constitutions arise in a number of different ways. At the non-democratic extreme of the spectrum, we may imagine a sovereign lawgiver laying down the constitution for all later generations. At the democratic extreme, we may imagine a constituent assembly elected by universal suffrage for the sole task of writing a new constitution. And there are all sorts of intermediate arrangements."

Australia held four constitutional conventions, one each in 1891, 1897, 1973, and 1998.

The Constituent Assembly of Bangladesh was the first and, to date, the only constitution-making body of Bangladesh, convened in 1972 by the government of Sheikh Mujibur Rahman following the country's independence. It comprised representatives elected in the national and provincial council elections of Pakistan held in 1970. As the assembly was formed with representatives elected under the Legal Framework Order, 1970, issued by Pakistan's then-military ruler and President Yahya Khan, several political parties and political leaders, including Maulana Abdul Hamid Khan Bhashani, Badruddin Umar, A.S.M. Abdur Rab, Farhad Mazhar, and others, have labeled this assembly as illegitimate. However, despite the controversies and opposition, Sheikh Mujib's uncompromising leadership enabled the Constituent Assembly to draft and enact the Constitution in less than a year. However, from the time of its drafting until today, the constitution has been often labelled as "fascist" and criticized for fostering autocracy and failing to adequately safeguard human rights.

In the aftermath of 2024 mass uprising, the interim government of Bangladesh is mulling over convening a new constituent assembly to draft a new inclusive democratic constitution, ensuring the inviolability of human dignity.

See also 2024 Bangladesh constitutional crisis.

As of May 2021 Chile is the most recently elected constitutional assembly. The 155 members of this assembly were elected between 15 and 16 May 2021. The assembly has gender parity (50% females and 50% males) and has 17 seats reserved for people belonging to indigenous peoples. The assembly is granted 12 months to draft a new constitution, which has to be ratified by referendum once written, with compulsory voting.