The Constitution of India is the supreme legal document of India, and the longest written national constitution in the world. 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 espouses constitutional supremacy (not parliamentary supremacy found in the United Kingdom, since it was created by a constituent assembly rather than Parliament) and was adopted with a declaration in its preamble. The Indian Constitution does not contain a provision to limit the powers of the parliament to amend the constitution. However, the Supreme Court in Kesavananda Bharati v. State of Kerala held that there were certain features of the Constitution so integral to its functioning and existence that they could never be cut out of the constitution(known as the 'Basic Structure' Doctrine).

The Government of India Act 1935, mainly drafted by Samuel Hoare, provided the basis for the constitution of India. The Constituent Assembly of India adopted the constitution on 26 November 1949 and became effective on 26 January 1950. It became 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 To ensure constitutional autochthony, its framers repealed prior acts of the British parliament, in Article 395.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 nitrogen-filled case at the Parliament Library Building in New Delhi.

In 1928, the All Parties Conference convened a committee in Lucknow to prepare the Constitution of India, which was known as the Nehru Report.

With the exception of scattered French and Portuguese exclaves, India was under the British rule from 1858 to 1947. From 1947 to 1950, the same legislation continued to be implemented as India was a dominion of United Kingdom for these three years, as most of the princely states were convinced by Jawaharlal Nehru, Sardar Patel and Louis Mountbatten to sign the articles of integration with India, and the British Government continued to be responsible for the external security of the country. Thus, the constitution of India repealed the Indian Independence Act 1947 and Government of India Act 1935 when it became effective on 26 January 1950. India ceased to be a dominion of the British Crown and became a sovereign, democratic republic with the constitution. Articles 5, 6, 7, 8, 9, 60, 324, 366, 367, 379, 380, 388, 391, 392, 393, and 394 of the constitution came into force on 26 November 1949, and the remaining articles became effective on 26 January 1950 which is celebrated every year in India as Republic Day.

The Indian constitution was drawn from a number of sources, mainly the Government of India Act 1935 which was significantly drafted by Samuel Hoare. Mindful of India's needs and conditions, its framers borrowed features of previous legislation such as the Government of India Act 1858, the Indian Councils Acts of 1861, 1892 and 1909, the Government of India Acts 1919 and 1935, and the Indian Independence Act 1947. The latter, which led to the creation of Pakistan, divided the former Constituent Assembly in two. Each new assembly had sovereign power to draft and enact a new constitution for the separate states.

The constitution was drafted by the Constituent Assembly, which was elected by elected members of the provincial assemblies. The 389-member assembly (reduced to 299 after the partition of India) took almost three years to draft the constitution holding eleven sessions over a 165-day period.