The Constitution of the Republic of Singapore is the supreme law of Singapore. A written constitution, the text which took effect on 9 August 1965 is derived from the Constitution of the State of Singapore 1963, provisions of the Federal Constitution of Malaysia made applicable to Singapore by the Republic of Singapore Independence Act 1965 (No. 9 of 1965, 1985 Rev. Ed.), and the Republic of Singapore Independence Act itself. The text of the Constitution is one of the legally binding sources of constitutional law in Singapore, the others being judicial interpretations of the Constitution, and certain other statutes. Non-binding sources are influences on constitutional law such as soft law, constitutional conventions, and public international law.

In the exercise of its original jurisdiction – that is, its power to hear cases for the first time – the High Court carries out two types of judicial review: judicial review of legislation, and judicial review of administrative acts. Although in a 1980 case the Privy Council held that the fundamental liberties in Part IV of the Constitution should be interpreted generously, Singapore courts usually adopt a philosophy of deference to Parliament and a strong presumption of constitutional validity, which has led to fundamental liberties being construed narrowly in certain cases. The courts also generally adopt a purposive approach, favouring interpretations that promote the purpose or object underlying constitutional provisions.

Article 4 of the Constitution expressly declares that it is the supreme law of the land. The Constitution also appears to satisfy Albert Venn Dicey's three criteria for supremacy: codification, rigidity, and the existence of judicial review by the courts. However, the view has been taken that it may not be supreme in practice and that Singapore's legal system is de facto characterized by parliamentary sovereignty.

There are two ways to amend the Constitution, depending on the nature of the provision being amended. Most of the Constitution's Articles can be amended with the support of more than two-thirds of all the Members of Parliament during the Second and Third Readings of each constitutional amendment bill. However, provisions protecting Singapore's sovereignty can only be amended if supported at a national referendum by at least two-thirds of the total number of votes cast. This requirement also applies to Articles 5(2A) and 5A, though these provisions are not yet operational. Article 5(2A) protects certain core constitutional provisions such as the fundamental liberties in Part IV of the Constitution, and Articles relating to the President's election, powers, maintenance, immunity from suit, and removal from office; while Article 5A enables the President to veto proposed constitutional amendments that directly or indirectly circumvent or curtail his discretionary powers. These provisions are not yet in force as the Government views the Elected Presidency as an evolving institution in need of further refinements.

The Malaysian courts have distinguished between the exercise of "constituent power" and "legislative power" by Parliament. When Parliament amends the Constitution by exercising constituent power, the amendment Act cannot be challenged as inconsistent with the Constitution's existing provisions. The Singapore position is unclear since this issue has not been raised before the courts. However, it is arguable that they are likely to apply the Malaysian position as the relevant provisions of the Constitution of Malaysia and the Singapore Constitution are in pari materia with each other. In addition, the High Court has rejected the basic structure or basic features doctrine developed by the Supreme Court of India, which means that Parliament is not precluded from amending or repealing any provisions of the Constitution, even those considered as basic.