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.

That total number was increased to 151 by the 1959 Delimitation Commission and the term of the House was five years[2] 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.

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.

[7] 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.

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

The Constitution also provided for a Parliamentary Commissioner for Administration (Ombudsman) who could investigate public grievances against government institutions and state officers and give redress.