In December 1905, after returning to Washington from their diplomatic travels Alice became engaged to Nicholas Longworth, a Republican U.S. House of Representatives member from Cincinnati, Ohio, who ultimately would rise to become Speaker of the House. The two had travelled in the same social circles for several years, but their relationship solidified during the Imperial Cruise. A scion of a socially prominent Ohio family, Nicholas Longworth was 14 years Alice's senior and had a reputation as a Washington, D.C., playboy. Their wedding took place the following February and was the social event of the season. It was attended by more than a thousand guests with many thousands gathered outside hoping for a glimpse of Princess Alice. The bride was dressed in a blue wedding dress and dramatically cut the wedding cake with a sword (borrowed from a military aide attending the reception). Immediately after the wedding, the couple left for a honeymoon that included a voyage to Cuba and a visit to the Longworths in Cincinnati. This was followed by travels to England and the Continent which included having dinners with many notables of the day: King Edward, Kaiser Wilhelm, Clemenceau, Whitelaw Reid, Lord Curzon, and William Jennings Bryan.
Alice Longworth's campaign against her husband caused a permanent chill in her marriage to Nick Longworth. During their marriage, Longworth carried on numerous affairs. As reported in Carol Felsenthal's biography of Alice, and in Betty Boyd Caroli's The Roosevelt Women, as well by TIME journalist Rebecca Winters Keegan, it was generally accepted knowledge in DC that Longworth also had a long, ongoing affair with Senator William Borah, and the opening of Longworth's diaries to modern historical researchers indicates that Borah was, by Longworth's own admission, the father of her daughter, Paulina Longworth (1925–1957).