The 1921 U.S. National Championships (now known as the US Open) was a tennis tournament that took place on the outdoor grass courts at three locations in the United States. The men's singles event was held from September 9 until September 19 at the Germantown Cricket Club in Philadelphia, while the women's singles and doubles events were held from August 15 until August 20 at the West Side Tennis Club in Forest Hills, New York. The men's and mixed doubles competitions took place at Longwood Cricket Club in Chestnut Hill, Massachusetts. It was the 41st staging of the U.S. National Championships and the second Grand Slam tennis event of the year. Although Molla Bjurstedt Mallory won the women's singles event for the sixth time in seven years, her victory was overshadowed by her second-round win over Suzanne Lenglen, who was making her only ever competitive appearance in the United States. Lenglen retired from the match after losing the first set, the only competitive loss of her career after World War I.

Finals

Men's singles

🇺🇸 Bill Tilden defeated 🇺🇸 Wallace F. Johnson 6–1, 6–3, 6–1

Women's singles

🇺🇸 Molla Bjurstedt Mallory defeated 🇺🇸 Mary Browne 4–6, 6–4, 6–2

Men's doubles

🇺🇸 Bill Tilden / 🇺🇸 Vincent Richards defeated 🇺🇸 Richard Norris Williams / 🇺🇸 Watson Washburn 13–11, 12–10, 6–1

Women's doubles

🇺🇸 Mary Browne / 🇺🇸 Louise Riddell Williams defeated 🇺🇸 Helen Gilleaudeau / 🇺🇸 Aletta Bailey Morris 6–3, 6–2

Mixed doubles

🇺🇸 Mary Browne / 🇺🇸 Bill Johnston defeated 🇺🇸 Molla Bjurstedt Mallory / 🇺🇸 Bill Tilden 3–6, 6–4, 6–3