Nicholas Longworth III (November 5, 1869 – April 9, 1931) was an American lawyer and politician who became Speaker of the United States House of Representatives. A Republican, he was elected to the Ohio Senate, where he initiated the successful Longworth Act of 1902, regulating the issuance of municipal bonds. As congressman for Ohio's 1st congressional district, he soon became a popular social figure of Washington, and married President Theodore Roosevelt's daughter Alice Lee. Their relationship became strained when he opposed her father in the Republican Party split of 1912. Longworth became Majority Leader of the House in 1923, and Speaker from 1925 to 1931.

In 1962, the Longworth House Office Building was named after him.

Longworth was the son of Nicholas Longworth II and Susan Walker. The Longworth family was an old, prominent, and wealthy family which dominated Cincinnati, Ohio. He had two younger sisters, Anna and Clara. Nicholas Longworth II was the son of Joseph Longworth and grandson of winemaker Nicholas Longworth I, both distinguished citizens of Cincinnati.

Nicholas Longworth III attended the Franklin School, a school for boys in Cincinnati, and then went on to attend Harvard College (Class of 1891), where he was a member of Delta Kappa Epsilon (Alpha chapter) and the Porcellian Club. He was a talented, but not necessarily an industrious student; one friend wrote about him: "His good head made it easy for him to get perfectly respectable marks without doing much of any work." After receiving his bachelor's degree from Harvard, he attended Harvard Law School for one year, but transferred to and received his law degree from Cincinnati Law School in 1894.

Longworth was a violinist, and on their first visit to Bayreuth, his wife Alice Lee Roosevelt reported that "Nick was really a musician and cared deeply for music...." Later she observed that "In Washington, Nick never had much time to play his violin, and in those days there were very few people to play with him. In Cincinnati there were the orchestra, the College of Music, and the Conservatory to draw on, and soon we were having musical parties, at least once, and often two or three times a week. ... We would all have dinner first, the musicians and a few others who cared for music, and afterwards lose no time getting started, by about nine at the latest. From then on music and yet more music until midnight and usually long after." In a letter to Longworth's sister Clara, Leopold Stokowski wrote "Your brother had a rare understanding of music. He penetrated directly into the spirit of music. It was his natural element."

Longworth began a law practice in Cincinnati after being admitted to the Ohio bar in 1894. His political career began with a position on the city's Board of Education in 1898.

As the protégé of Republican boss George B. Cox, Longworth was elected to the Ohio General Assembly, serving in the Ohio House of Representatives in 1899 and 1900, then in the State Senate from 1901 to 1903. In 1902 he was instrumental in writing and passing the Longworth Act, a bill regulating the issuance of municipal bonds, which has been labeled "one of the most successful laws in Ohio's history." Longworth was elected to the United States House of Representatives from the First Congressional District of Ohio which included the city of Cincinnati and the surrounding counties.

Longworth, a bachelor when he entered Congress, married Alice Lee Roosevelt, the daughter of President Theodore Roosevelt, on February 17, 1906, in a White House wedding that received widespread public attention In 1925, Roosevelt gave birth to a daughter named Paulina Longworth, who was conceived from her affair with Senator William Borah. One family friend said of Paulina, "everybody called her 'Aurora Borah Alice.'" Biographers and historians have concluded that though Longworth was delighted with Paulina's birth and doted on her, he almost certainly knew that Borah was her father. Longworth also had affairs, but the couple remained married, though Alice's support for the progressive movement while Longworth sided with the conservative wing of the Republican Party caused a political rift between them.