Norfolk County (/ˈnɔːrfək/ NOR-fək) is located in the U.S. state of Massachusetts. At the 2020 census, the population was around 725,981. Its county seat is Dedham. The county was named after the English county of the same name. Two towns, Cohasset and Brookline, are exclaves. Norfolk County is included in the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area. Norfolk County is the 24th highest-income county in the United States with a median household income of $107,361. It is the wealthiest county in Massachusetts.

One of the original counties of the Massachusetts Bay Colony created on May 10, 1643, was called Norfolk, and is unrelated to the current Norfolk County. It covered territory in what is now New Hampshire, and was abolished on September 18, 1679, when King Charles II separated the Colony of New Hampshire from Massachusetts.

Shortly after the Constitution of Massachusetts was adopted on October 25, 1780, a number of towns in Suffolk County, of which Dedham was then a part, elected delegates to a convention to decide upon some policy relative to the division of the county. The convention met at Gay's Tavern in Dedham on December 12, 1780, and adopted a resolution to the effect that the towns of Bellingham, Dedham, Foxborough, Franklin, Medfield, Medway, Needham, Stoughton, Stoughtonham, Walpole, and Wrentham, along with the Middlesex County towns of Holliston, Hopkinton, Natick, and Sherborn ought to be formed into a new county with Medfield as the shiretown. The Great and General Court did not look favorably upon the resolution, however, and no new county was then created.

Norfolk County, Massachusetts was created on June 20, 1793. Legislation passed in March which separated off all the towns in Suffolk County except Boston and Chelsea (which at the time included what are now Revere and Winthrop). Dedham was designated as the "shire town". Hingham and Hull petitioned to remain in Suffolk County and on the day the law was to take effect, their removal to Norfolk County was repealed, making Cohasset an exclave. In 1803, they were moved into Plymouth County, Massachusetts.

On June 22, 1797, the town of Natick was given to Middlesex County, and the town of Needham was transferred from Middlesex to Norfolk. The towns of Dorchester and Roxbury were part of Norfolk County when it was created but, as Boston annexed each town in stages from 1804 to 1912, they became part of Suffolk County again, leaving Brookline separated from the rest of Norfolk County. No other changes have been made to the territory of Norfolk County, other than new municipalities being created within its boundaries, and minor border adjustments.

Norfolk County is the birthplace of four Presidents of the United States (John Adams, John Quincy Adams, John F. Kennedy, and George H. W. Bush), resulting in the moniker "County of Presidents."

Following the creation of the county, Gay's Tavern was the site of a Court of General Sessions on August 25, 1794. They ordered that the committee on buildings proceed with collecting materials for building a jail. On the last day of September following this order, the court accepted from Timothy Gay the gift of a parallelogram lot of land to erect the Norfolk County Jail next to his tavern.

It was replaced by a new Norfolk County Jail in 1817. The two story stone building was built in 1817 and was 33' square. Part of the jail was torn down in 1851 to erect a central, octagonal portion and two wings. It resulted in a building with the shape of a Latin cross, and featured Gothic Revival windows. The three tiers of cells radiated out like spokes from the central guardroom. There were two hangings in the central rotunda: George C. Hersey on August 8, 1862, and James H. Costley on June 25, 1875.