Killingworth is a town in Middlesex County, Connecticut, United States. The town's name can easily be confused with another Connecticut town, Killingly; or a Vermont ski area, Killington. The population was 6,455 as of July 1, 2015.

Killingworth was established from the area called Hammonasset, taken from the local Native American tribe of the same name. The area originally incorporated the town of Clinton, which were separated along ecclesiastical borders. Part of New London County prior to May 1785, Killingworth was then included in the newly formed Middlesex County, where it remains today.

It was named after Kenilworth, England in honor of one of the first settlers, Edward Griswold. Kenilworth's name was more similar to "Killingworth" during the American colonial period, and over time the pronunciation and spelling drifted towards the modern one. Coincidentally, there is a town and village in England called Killingworth and Killingworth Village in the county of Tyne and Wear, which do not seem to have any connection with Killingworth, Connecticut.

In the late 17th century, Killingworth became the birthplace of what would eventually become Yale University. The Rev. Abraham Pierson, the college's first president, taught some of the first classes in his Killingworth home - which is actually in present-day Clinton, Connecticut. However, in 1701, the college's first official home was constructed in Old Saybrook on the peninsula known as Saybrook Point. Eventually the school was moved to its present-day home in New Haven.

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