Delaware ( DEL-ə-wair) is a state in the Mid-Atlantic and South Atlantic states region of the United States. It borders Maryland to its south and west, Pennsylvania to its north, New Jersey to its northeast, and the Atlantic Ocean to its east. The state's name derives from the adjacent Delaware Bay, which in turn was named after Thomas West, 3rd Baron De La Warr, an English nobleman and the Colony of Virginia's first colonial-era governor.

Delaware occupies the northeastern portion of the Delmarva Peninsula, and some islands and territory within the Delaware River. It is the second-smallest and sixth-least populous state, but also the sixth-most densely populated. Delaware's most populous city is Wilmington, and the state's capital is Dover, the second-most populous city in Delaware. The state is divided into three counties, the fewest number of counties of any of the 50 U.S. states; from north to south, the three counties are: New Castle County, Kent County, and Sussex County. The southern two counties, Kent and Sussex counties, historically have been predominantly agrarian economies. New Castle is more urbanized and is considered part of the Delaware Valley metropolitan statistical area that surrounds Philadelphia. Delaware is considered part of the Southern United States by the U.S. Census Bureau, but the state's geography, culture, and history are a hybrid of the Mid-Atlantic and Northeastern regions of the country.

Before the Delaware coastline was explored and developed by Europeans in the 16th century, the state was inhabited by several Native American tribes, including the Lenape in the north and Nanticoke in the south. The state was first colonized by Dutch traders at Zwaanendael, near present-day Lewes, Delaware, in 1631. Delaware was one of the Thirteen Colonies that participated in the American Revolution against Great Britain, which established the United States as an independent nation. On December 7, 1787, Delaware was the first state to ratify the Constitution of the United States, earning it the nickname "The First State".

Since the turn of the 20th century, Delaware has become an onshore corporate haven whose corporate laws are deemed appealing to corporations; over half of all New York Stock Exchange-listed corporations and over three-fifths of the Fortune 500 are legally incorporated in Delaware. Over 90% of all U.S. based companies that went public in 2021 incorporated themselves in Delaware.