Legislative power is vested in the National Assembly, a unicameral legislature. The Assembly is composed of 299 members serving four-year terms.

Out of 299 members, 245 are elected by popular vote from local constituencies, while the remaining 54 members obtain their seats through a proportional representation system in which seats are allocated to each political party that has gained 3 percent or more of all valid votes or five or more seats in the local constituency election. The system is aimed at reflecting the voices of people from different walks of life while enhancing the expertise of the Assembly.

To be eligible for election, a candidate must be at least 25 years of age. One candidate from each electoral district is selected by majority vote.

An Assembly member is not held responsible outside the Assembly for any opinions expressed or votes cast in the legislative chamber. During a session of the Assembly, no Assembly member may be arrested or detained without consent of the Assembly except in the case of a flagrant criminal act.

In case of apprehension or detention of an Assembly member prior to the opening of a session, the member must be released during the session upon the request of the Assembly.

Two types of legislative sessions are provided for, regular and special. The regular session is convened once a year from September through December and special sessions may be convened upon the request of the President or one-fourth or more of the members of the Assembly. The period of a regular session is limited to 100 days and to 30 days for special sessions. If the President requests the convening of a special session, he must clearly specify the period of the session and the reasons for the request.

Except as otherwise provided in the Constitution or law, the attendance of more than one half of the entire Assembly members, and the concurrent vote of more than one half of the Assembly members present, are necessary to make the decisions of the National Assembly binding. In the case of a tie vote, the matter is considered to be rejected by the Assembly. Legislative meetings are open to the public, but this rule may be waived with the approval of more than one half of the members present or when the Speaker deems it necessary to do so in the interest of national security.

The National Assembly is vested with a number of functions under the Constitution, the foremost of which is making laws. Other functions of the Assembly include approval of the national budget, matters related to foreign policy, declaration of war, and the stationing of Korean troops abroad or of foreign forces within the country, inspecting or investigating specific matters of state affairs and impeachment.

An impeachment motion against an appointed official can come to a vote if approved by at least one third of the Assembly. The impeachment passes if it receives the backing of a majority of the Assembly members. A motion of impeachment against the President requires a majority vote from the Assembly to proceed. The motion passes only if two-thirds or more of the entire Assembly approves.

The Assembly elects one Speaker and two Vice Speakers, who serve for two-year terms. The Speaker presides over plenary sessions and represents the legislature while supervising its administration. In the absence of the Speaker, the Vice Speakers shall act for the Speaker.