The presidency of Theodore Roosevelt started on September 14, 1901, when Theodore Roosevelt became the 26th president of the United States upon the assassination of President William McKinley, and ended on March 4, 1909. Roosevelt had been the vice president for only 194 days when he succeeded to the presidency. A Republican, he ran for and won by a landslide a four-year term in 1904. He was succeeded by his protégé and chosen successor, William Howard Taft.

Theodore Roosevelt Jr., often referred to as Teddy or by his initials, T. R., was an American politician, statesman, conservationist, naturalist, and writer who served as the 26th president of the United States from 1901 to 1909. He previously held various positions in New York politics, rising up the ranks to serve as the state's 33rd governor for two years. He later served as the 25th vice president under President William McKinley for six months in 1901, assuming the presidency after McKinley's assassination. As president, Roosevelt emerged as a leader of the Republican Party and became a driving force for anti-trust and Progressive policies.