The Slavery Abolition Act 1833 (3 & 4 Will. 4. c. 73) was an act of the Parliament of the United Kingdom which abolished slavery in the British Empire by way of compensated emancipation. The act was legislated by Whig Prime Minister Charles Grey, 2nd Earl Grey's reforming administration, and it was enacted by ordering the British government to purchase the freedom of all slaves in the British Empire, and by outlawing the further practice of slavery in the British Empire. The Act explictly delineated 19 separate pots of compensation covering the Caribbean, South Africa, and Mauritius. Although Britain, Canada, Australia, and New Zealand were technically included, these had relatively few slaves at this time for other reasons. India was excluded. Around 800,000 freed slaves were attested in the claims process.^{[citation needed]}

While the 1833 Act was a landmark, it did not end slavery throughout the entire British sphere of influence. The Act explicitly excluded territories like British India, where slavery was addressed separately by the Indian Slavery Act, 1843. In regions colonized later, such as Nigeria, the abolition of pre-existing local systems of slavery was a gradual process that extended into the early 20th century. Furthermore, in British protectorates, which retained their own local laws, the institution persisted for much longer. For example, slavery in Bahrain was not legally abolished until 1937.

The Act was technically repealed in 1998 as part of a restructuring of the entirety of English statute law, though slavery remains abolished.

Slavery had been abolished in England by 1772. In May 1772, Lord Mansfield's judgment in the Somerset case emancipated a slave who had been brought to England from Boston in the Province of Massachusetts Bay, and thus helped launch the movement to abolish slavery throughout the British Empire. The case ruled that slavery had no legal status in England as it had no common law or statutory law basis, and as such someone could not legally be a slave in England. However, many campaigners, including Granville Sharp, took the view that the ratio decidendi of the Somerset case meant that slavery was unsupported by law within England and that no ownership could be exercised on slaves entering English or Scottish soil. Ignatius Sancho, who in 1774 became the second recorded black person to vote in a British general election — the first being John London — wrote a letter in 1778 that opens in praise of Britain for its "freedom, and for the many blessings I enjoy in it", before criticizing the actions towards his black brethren in parts of the Empire such as the West Indies.

By 1783, an anti-slavery movement to abolish the slave trade throughout the Empire had begun among the British public, with the Society for Effecting the Abolition of the Slave Trade being established in 1787. The Wedgwood anti-slavery medallion by Josiah Wedgwood was, according to the BBC, "the most famous image of a black person in all of 18th-century art". Fellow abolitionist Thomas Clarkson wrote: "Of the ladies several wore them in bracelets, and others had them fitted up in an ornamental manner as pins for their hair. At length, the taste for wearing them became general; and thus fashion, which usually confines itself to worthless things, was seen for once in the honourable office of promoting the cause of justice, humanity and freedom."

Spurred by an incident involving Chloe Cooley, a slave woman brought to Canada by an American loyalist, the Lieutenant-Governor of Upper Canada, John Graves Simcoe, tabled the Act Against Slavery in 1793. Passed by the local Legislative Assembly, it was the first legislation to outlaw the slave trade in a part of the British Empire. By the late 18th century, Britain was simultaneously the largest slave trader and centre of the largest abolitionist movement. William Wilberforce had written in his diary in 1787 that his great purpose in life was to suppress the slave trade before waging a 20-year fight on the industry.

Parliament passed the Slave Trade Act 1807 (47 Geo. 3 Sess. 1. c. 36), which outlawed the international slave trade, but not slavery itself. The legislation was timed to coincide with the expected Act Prohibiting Importation of Slaves by the United States, Britain's chief rival in maritime commerce. This legislation imposed fines that did little to deter slave trade participants. Abolitionist Henry Brougham realized that trading had continued, and as a new MP successfully introduced the Slave Trade Felony Act 1811 (51 Geo. 3. c. 23) which at last made the overseas slave trade a felony throughout the empire. The Royal Navy established the West Africa Squadron to suppress the Atlantic slave trade by patrolling the coast of West Africa. It did suppress the slave trade, but did not stop it entirely. Between 1808 and 1860, the West Africa Squadron captured 1,600 slave ships and freed 150,000 Africans. They resettled many in Jamaica and the Bahamas. Britain also used its influence to coerce other countries to agree to treaties to end their slave trade and allow the Royal Navy to seize their slave ships.

Between 1807 and 1823, abolitionists showed little interest in abolishing slavery itself. Eric Williams presented economic data in Capitalism and Slavery to show that the slave trade itself generated only small profits compared to the much more lucrative sugar plantations of the Caribbean, and therefore slavery continued to thrive on those estates. However, from 1823 the British Caribbean sugar industry went into terminal decline, and the British parliament no longer felt they needed to protect the economic interests of the West Indian sugar planters.