The Mystic massacre – also known as the Pequot massacre and the Battle of Mystic Fort – took place on May 26, 1637 during the Pequot War, when a force from the Connecticut Colony under Captain John Mason and their Narragansett and Mohegan allies set fire to the Pequot Fort near the Mystic River. They shot anyone who tried to escape the wooden palisade fortress and killed most of the village. There were between 400 and 700 Pequots killed during the attack; the only Pequot survivors were warriors who were away in a raiding party with their sachem Sassacus.

The Pequots were the dominant Native American tribe in the southeastern portion of the Connecticut Colony, and they had long been enemies of the neighboring Mohegan and Narragansett tribes. The New England colonists established trade with all three tribes, exchanging European goods for wampum and furs. The Pequots eventually allied with the Dutch colonists, while the Mohegans and others allied with the New England colonists.

A trader named John Oldham was murdered and his trading ship looted by Pequots, and retaliation raids ensued by Colonists and their Native American allies. On April 23, 1637, 200 Pequot warriors attacked the village of Wethersfield killing six men and three women, all unarmed noncombatants. This was a major turning point in the Pequot war as it enraged the settlers that the warriors would kill civilians and led to increased support for the Pequot War among colonists. According to Katherine Grandjean, the Great Colonial Hurricane of 1635 damaged the corn and other crop harvests of that year, making food supplies scarce and creating competition for winter food supplies. This in turn increased the tensions between the Pequots and Colonists who were ill-prepared to face periods of famine.

The Connecticut towns raised a militia commanded by Captain John Mason consisting of 90 men, plus 70 Mohegans under sachems Uncas and Wequash. Twenty more men under Captain John Underhill joined him from Fort Saybrook. At the same time, Pequot sachem Sassacus took a few hundred warriors and set out to attack Hartford, Connecticut.^{[citation needed]}

Captain Mason recruited more than 200 Narragansett and Niantic Natives to join his force. On the night of May 26, 1637, the Colonial and Native forces arrived at the fortified Pequot village, which was on a low hill near the Mystic River. The large village was surrounded by a palisade with only two exits. The Colonial forces first attempted a surprise attack but they withdrew after stiff resistance from the Pequots.

In response, Mason ordered that the village should be set ablaze and its two exits blocked off. As the fire raged, many trapped Pequots were shot as they attempted to escape by climbing over the palisade; those men, women, and children who did get out were killed by Narragansett fighters. Captain Underhill described the scene and his participation:

"Captaine Mason entring into a Wigwam, brought out a fire-brand, after hee had wounded many in the house, then hee set fire on the West-side where he entred, my selfe set fire on the South end with a traine of Powder, the fires of both meeting in the center of the Fort blazed most terribly, and burnt all in the space of halfe an houre; many couragious fellowes were unwilling to come out, and fought most desperately through the Palisadoes, so as they were scorched and burnt with the very flame, and were deprived of their armes, in regard the fire burnt their very bowstrings, and so perished valiantly: mercy they did deserve for their valour, could we have had opportunitie to have bestowed it; many were burnt in the Fort, both men, women, and children, others forced out, and came in troopes to the Indians, twentie, and thirtie at a time, which our souldiers received and entertained with the point of the sword; downe fell men, women, and children, those that scaped us, fell into the hands of the Indians, that were in the reere of us; it is reported by themselves, that there were about foure hundred soules in this Fort, and not above five of them escaped out of our hands."

Mohegans would collect the heads of fallen Pequots, taking scalps as war trophies. Hundreds of Pequots were killed; the colonists reported that only five village occupants escaped while seven were taken prisoner.