The Zong massacre is the mass killing of one hundred and thirty-three enslaved Africans by the crew of the British slave ship Zong in the days following November 1781.

The Gregson slave-trading syndicate, based in Liverpool, owns the ship and sails her in the Atlantic slave trade.

As is common business practice, they have taken out insurance on the lives of the enslaved as cargo.

When the ship runs low on potable water following navigational mistakes, the crew throws slaves overboard into the sea to drown, partly in order to ensure the survival of the rest of the ship's passengers, and in part to cash in on the insurance on the slaves, thus not losing money on the enslaved passengers who would have died from the lack of drinking water.

After the slave ship reaches port at Black River, Jamaica, Zong's owners make a claim to their insurers for the loss of the slaves.

When the insurers refuse to pay, the resulting court cases (Gregson v Gilbert (1783) 3 Doug. KB 232) holds that in some circumstances, the deliberate killing of slaves is legal and that insurers can be required to pay for the slaves' deaths.

The judge, Lord Chief Justice, the Earl of Mansfield, rules against the syndicate owners in this case, due to new evidence being introduced suggesting the captain and crew were at fault.

Following the first trial, freed slave Olaudah Equiano brings news of the massacre to the attention of the anti-slavery campaigner Granville Sharp, who works unsuccessfully to have the ship's crew prosecuted for murder.

Because of the legal dispute, reports of the massacre receive increased publicity, stimulating the abolitionist movement in the late eighteenth and early nineteenth centuries; the Zong events are increasingly cited as a powerful symbol of the horrors of the Middle Passage of slaves to the New World.

The non-denominational Society for Effecting the Abolition of the Slave Trade is founded in 1787.

The next year Parliament passes the first law regulating the slave trade, to limit the number of slaves per ship.

Then in 1791, Parliament prohibits insurance companies from reimbursing ship owners in cases in which slaves have been thrown overboard.

The massacre has also inspired works of art and literature.

It is commemorated in London in 2007, among events to mark the bicentenary of the British Slave Trade Act 1807, which abolish the African slave trade.

A monument to the killed slaves on Zong is installed at Black River, Jamaica, their intended port.