The Constitution of India (Bhāratīya Saṃvidhāna) is the supreme law of India. The document lays down the framework that demarcates fundamental political code, structure, procedures, powers, and duties of government institutions and sets out fundamental rights, directive principles, and the duties of citizens. It is the longest written constitution of any country on Earth.

It imparts constitutional supremacy and was adopted by its people with a declaration in its preamble. Parliament cannot override the constitution.

It was adopted by the Constituent Assembly of India on 26 November 1949 and became effective on 26 January 1950. The constitution replaced the Government of India Act 1935 as the country's fundamental governing document, and the Dominion of India became the Republic of India. To ensure constitutional autochthony, its framers repealed prior acts of the British parliament in Article 395. India celebrates its constitution on 26 January as Republic Day.

The constitution declares India a sovereign, socialist, secular, and democratic republic, assures its citizens justice, equality and liberty, and endeavours to promote fraternity.

The original 1950 constitution is preserved in a helium-filled case at the Parliament House in New Delhi. The words "secular" and "socialist" were added to the preamble in 1976 during the Emergency.