The Tamil Nadu Legislative Assembly is the unicameral legislature of the Indian state of Tamil Nadu. It has a strength of 234 members, all of whom are democratically elected using the first-past-the-post system. The presiding officer of the Assembly is the Speaker. The term of the Assembly is five years, unless dissolved earlier.

Since Tamil Nadu has a unicameral legislature, the terms Tamil Nadu Legislature and Tamil Nadu Legislative Assembly are almost synonymous and are often confused. However, they are not one and the same. The Tamil Nadu Legislature is the legislative body, while the Tamil Nadu Legislative Assembly is a part of it. The Tamil Nadu Legislative Assembly, along with the Governor of Tamil Nadu, constitutes the Tamil Nadu Legislature.

The present state of Tamil Nadu is a residuary part of the erstwhile Madras Presidency and was formerly known as Madras State. The first legislature of any sort for the Presidency was the Madras Legislative Council, which was set up as a non-representative advisory body in 1861. In 1919, direct elections were introduced with the introduction of diarchy under the Government of India Act 1919. Between 1920 and 1937, the Legislative Council was a unicameral legislature for the Madras Presidency. The Government of India Act 1935 abolished diarchy and created a bicameral legislature in the Madras Presidency. The Legislative Assembly became the Lower House of the Presidency.

After the Republic of India was established in 1950, the Madras Presidency became the Madras State, and the bicameral setup continued. The Madras State's assembly strength was 375, and the first assembly was constituted in 1952. The current state was formed in 1956 after the reorganisation of states, and the strength of the assembly was reduced to 206. Its strength was increased to the present 234 in 1965. Madras State was renamed Tamil Nadu in 1969, and subsequently, the assembly came to be called the Tamil Nadu Legislative Assembly. The Legislative Council was abolished in 1986, making the legislature a unicameral body and the assembly its sole chamber.

The present Sixteenth Legislative Assembly was constituted on 3 May 2021. It was constituted after the 2021 assembly election, which resulted in the Dravida Munnetra Kazhagam (DMK)-led front winning and forming the government. The next election will take place in 2026.

The first legislature of any kind to be established in Madras was the Madras Legislative Council in 1861. First established as a non-representative advisory body, it saw the introduction of elected members in 1892. The Indian Councils Act 1909 (popularly called the "Minto-Morley Reforms") officially introduced indirect election of members to the Council. In 1919, direct elections were introduced with the introduction of diarchy under the Government of India Act 1919. Between 1920 and 1937, the Legislative Council was a unicameral legislature for the Madras Presidency. The Government of India Act 1935 abolished diarchy and created a bicameral legislature in Madras province. The Legislature consisted of the Governor and two legislative bodies: a Legislative Assembly and a Legislative Council. The Assembly was the lower house and consisted of 215 members, who were further classified into general members and reserved members representing special communities and interests:

The presiding officer of the Assembly was called the Speaker of the Assembly.

The first legislative assembly election in the presidency was held in February 1937. The Indian National Congress obtained a majority by winning 159 of 215 seats. C. Rajagopalachari became the first elected chief minister of the Presidency under the provincial autonomy system guaranteed by the Government of India Act 1935. The first assembly was constituted in July 1937. Bulusu Sambamurti and A. Rukmani Lakshmipathi were elected as the Speaker and Deputy Speaker respectively.