The Constitution of the Democratic Socialist Republic of Sri Lanka (Sinhala: ශ්‍රී ලංකා ආණ්ඩුක්‍රම ව්‍යවස්ථාව, romanized: Śrī Laṅkā āndukrama vyavasthāva, Tamil: இலங்கைச் சனநாயக சோசலிசக் குடியரசின் அரசமைப்பு, romanized: Ilaṅkaic caṉanāyaka cōcalicak kuṭiyaraciṉ aracamaippu) has been the constitution of the island nation of Sri Lanka since its original promulgation by the National State Assembly on 7 September 1978. As of October 2022^[update] it has been formally amended 21 times.

It is Sri Lanka's second republican constitution, replacing the Sri Lankan Constitution of 1972, its third constitution since the country gained independence within the British Commonwealth as the Dominion of Ceylon in 1948, and its fourth constitution overall.

Under the Soulbury Constitution, which consisted of The Ceylon Independence Act, 1947 and The Ceylon (Constitution and Independence) Orders in Council 1947, Sri Lanka was then known as Ceylon. The Soulbury Constitution provided a parliamentary form of Government for Ceylon and for a Judicial Service Commission and a Public Service Commission. Minority rights were safeguarded by Article 29(2) of the Constitution. The governor-general, the representative of the monarch of Ceylon, the Senate and the House of Representatives exercised legislative power. The House of Representatives consisted of 101 Members, of which 95 were elected by universal suffrage and 6 were nominated by the Governor-General. That total number was increased to 151 by the 1959 Delimitation Commission and the term of the House was five years The S. W. R. D. Bandaranaike Government set up a Joint Select Committee of the Senate and the House of Representatives to consider a revision of the Constitution on 10 January 1958 but the committee was unable to come to a final conclusion on account of the propagation of Parliament on 23 May 1959. A similar attempt by the Dudley Senanayake Government failed due to such a propagation on 22 June 1968 too. The Senate consisted of 30 members (15 elected by the House and 15 by the Governor-General) was abolished on 2 October 1971.

Amendments

Sirimavo Bandaranaike came to office as the world's first woman Prime Minister in May 1970. Her United Front Government used the parliament as a Constituent Assembly and drafted a new Republican Constitution. It was promulgated on 22 May 1972. This Constitution provided for a unicameral legislature named the National State Assembly with a term of office of 6 years and Sovereignty was entirely vested in it. A nominal President with a term of office of 4 years was appointed as the Head of State by the Prime Minister, Head of the Cabinet of Ministers responsible to the National State assembly. Ceylon was replaced by republic of Sri Lanka (Resplendent Island). This constitution containing a declaration of fundamental rights and freedom was amended on 11 February 1975 to change the basis of delimitation of constituencies from 75,000 persons per electorate to 90,000 persons. J. R. Jayewardene who came to office in July 1977 with a five-sixths majority passed the second amendment to the 1972 Constitution on 4 October 1977, which made the presidency an Executive post.

Clause 3 of the amendment specifically stated that:

"The person holding the office of Prime Minister at the commencement of this amendment shall assume the office of the President of the Republic and shall exercise and perform the powers and functions pertaining to that office."

Under its provisions, then Prime Minister Jayawardene automatically took office as the first Executive President of Sri Lanka on 4 February 1978.