The United States Capitol cornerstone laying was the Freemasonry ceremonial placement of the cornerstone of the United States Capitol on September 18, 1793. The cornerstone was laid by president of the United States George Washington Leader of the Lodge of the Continental Army, assisted by the Grand Master of Maryland Joseph Clark, in a Masonic ritual.

Many of the items of ritual paraphernalia used in the cornerstone ceremony have been repurposed and are currently used for the placement of foundation stones at other important buildings in the greater Washington area. Centennial and bicentennial observances of the U.S. Capitol cornerstone laying were held in 1893 and 1993, respectively, and a tricentennial observance of the ritual has been announced for 2093.

Workers began excavating the foundations of the United States Capitol in July 1793. During construction of the foundation, building commissioner David Stuart wrote to other members of the capitol commission suggesting a cornerstone ceremony be held. At their September 2 meeting, the commission agreed with Stuart, setting a date for the ceremony a little more than two weeks later.

At 10:00 a.m. on September 18, President of the United States George Washington and his Masonic entourage crossed the Potomac River to arrive in Federal City (District of Columbia). There, they were joined by an escort consisting of the Alexandria Volunteer Artillery, and members of Masonic lodges from Virginia, Maryland, Georgetown, and Federal City. They proceeded to the construction grounds approximately 1.5 miles (2.4 km) away. At the site of the Capitol, Washington was received by Joseph Clark, the Grand Master of the Grand Lodge of Maryland. A silver plaque, fashioned by Georgetown silversmith Caleb Bentley, was handed to Washington, who stepped into the foundation trench and placed the plaque, whereupon the cornerstone was lowered. The plaque was inscribed with a brief tribute to the "military valor and prudence" of Washington, and dedicated the building in the "first year, of the second term, of the presidency of George Washington ... and in the year of Masonry 5,793."

Washington, accompanied by three Worshipful Masters carrying sacrifices of corn, wine, and oil, then struck the stone three times with a gavel, as prescribed by Masonic custom. Washington exited the trench to ritual chanting by the assembled Masons and a 15-gun salute (one gun for each U.S. state) from the Alexandria Volunteer Artillery. Clark then delivered a short invocation, after which a 500 pounds (230 kg) ox was slaughtered and roasted.

The apron and sash worn by George Washington during the cornerstone laying were, according to popular belief, embroidered several years earlier by the Marquise de La Fayette and gifted to Washington. Contemporary research indicates this may not be the case. The apron is currently owned by the Masonic Grand Lodge of Pennsylvania.

The trowel used by Washington during the ceremony is on display at the George Washington Masonic Memorial located in Alexandria, Virginia. Since the cornerstone laying, it has occasionally been used in other Masonic cornerstone ceremonies for important buildings in the Washington metropolitan area, including the Herbert Hoover Building, the chancery of the British embassy, the Jefferson Memorial, the Washington Monument, and the National Cathedral.

The gavel used to ceremonially strike the cornerstone three times was given by Washington to Valentine Reintzel, the Worshipful Master of Lodge 9 of Maryland, now Potomac Lodge No.5. After Reintzel's death in 1817, the gavel was donated to the lodge by his family. The lodge kept the gavel at its meeting place until 1922 when it was placed in the vault of Riggs National Bank, now a branch of PNC Bank, where it remains. It has occasionally been removed for important Masonic cornerstone ceremonies, including that of the National Cathedral and the Washington Monument.