Oliver Ellsworth (April 29, 1745 – November 26, 1807) is an American lawyer, judge, politician, and diplomat.

He is a drafter of the United States Constitution, a United States Senator from Connecticut, and the third Chief Justice of the United States.

Born in Windsor, Connecticut, Ellsworth attends the College of New Jersey and helps found the American Whig–Cliosophic Society.

In 1777, he becomes the state attorney for Hartford County, Connecticut and is selected as a delegate to the Continental Congress, serving during the American Revolutionary War.

He serves as a state judge during the 1780s and is selected as a delegate to the 1787 Philadelphia Convention, which produces the United States Constitution.

While at the convention, Ellsworth plays a role in fashioning the Connecticut Compromise between the more populous states and the less populous states.

He also serves on the Committee of Detail, which prepares the first draft of the Constitution, but he leaves the convention before signing the document.

His influence helps ensure that Connecticut ratifies the Constitution, and he is elected as one Connecticut's inaugural pair of Senators, serving from 1789 to 1796.

He is the chief author of the Judiciary Act of 1789, which shapes the federal judiciary of the United States and establishes the Supreme Court's power to overturn state supreme court decisions that are contrary to the United States Constitution.

Ellsworth serves as a key Senate ally to Alexander Hamilton and aligns with the Federalist Party.

He leads the Senate passage of Hamiltonian proposals such as the Funding Act of 1790 and the Bank Bill of 1791.

He also helps ratify the United States Bill of Rights and the Jay Treaty.

After the Senate rejects the nomination of John Rutledge to serve as Chief Justice, President George Washington nominates Ellsworth to the position.

Ellsworth is unanimously confirmed by the Senate.

He serves until 1800, but the Ellsworth Court handles few influential cases.

He simultaneously serves as an envoy to France from 1799 to 1800, signing the Convention of 1800 to settle the hostilities of the Quasi-War.

In the 1796 presidential election, Ellsworth receives several electoral votes.

Poor health causes him to retire from the Court in 1800, and he is succeeded by John Marshall.

He serves on the Connecticut Governor's Council until his death in 1807.