Old Saybrook is a town in Middlesex County, Connecticut, part of the Lower Connecticut River Valley Planning Region. The population was 10,481 at the 2020 census. It contains the incorporated borough of Fenwick, and the census-designated places of Old Saybrook Center and Saybrook Manor.

Dutch settlers established a short-lived factory at Old Saybrook in 1624, shortly after establishing their first settlement at Governors Island. The trading post was named Kievits Hoek, or "Plover's Corner". Kievits Hoek was soon abandoned as the Dutch consolidated settlement at New Amsterdam. In 1633, Fort Goede Hoop (Huys de Goede Hoop) was established at Hartford.

The Pequot siege of Saybrook Fort took place from September 1636 to March 1637 during the Pequot War.

In August 1636, Massachusetts Bay attacked Manisses, Pequot, and Western Niantic villages, and the Pequot retaliation fell on the settlers at Saybrook. The Pequots killed and wounded more than 20 settlers at and near Saybrook Fort over a period of eight months. The settlers were attacked when they ventured far from their palisade, and the Pequots destroyed their provisions and burned warehouses, while attempting to interrupt river traffic to Windsor, Wethersfield, and Hartford. The siege of Saybrook escalated the Pequot War in Connecticut Colony and indirectly resulted in the attack and destruction of Mistick Fort in May 1637.

The Saybrook Colony was established in late 1635 at the mouth of the Connecticut River in Old Saybrook. John Winthrop, the Younger, son of the Governor of the Massachusetts Bay Colony, was designated governor by the group that claimed possession of the land via a deed of conveyance from Robert Rich, 2nd Earl of Warwick. Winthrop was aided by Colonel George Fenwick and Captain Lion Gardiner. The new colony struggled because the investors who had planned to settle there were supporters of Oliver Cromwell, and they remained in England during the English Civil War. In 1644, Fenwick agreed to merge the colony with the more vibrant Connecticut Colony up the Connecticut River near Hartford, and they purchased the land and fort from him.

The design of the Flag of Connecticut comes from the seal of Saybrook Colony. The seal was brought from England by Colonel George Fenwick, and depicted 15 grapevines and a hand in the upper left corner with a scroll reading "Qui Transtulit Sustinet", "He who transplanted sustains".

In 1647, Major John Mason assumed command of the fort at Saybrook Colony, which controlled the main trade and supply route to the upper Connecticut River valley. The fort promptly and mysteriously burned to the ground, but another improved fort was quickly built nearby on the Battery Mound. He spent the next 12 years there and also served as commissioner of the United Colonies, the chief military officer, magistrate, and peacekeeper. In 1659, almost all settlers from Saybrook under the leadership of Mason purchased land from Uncas, sachem of the Mohegan tribe; they then moved to and founded Norwich, Connecticut. This second fort was abandoned after 200 years and the battery mound remained until 1871, when the Valley Railroad leveled it and other hills on the point to provide needed fill for their tracks across the north and south coves.

In 1661, Saybrook residents Margaret Jennings and her husband Nicholas were accused of causing the deaths of Marie Marvin and others. The trial resulted in a finding that they were probably witches, but the proof was not sufficient to execute them.