Theodore Roosevelt Jr. ( ROH-zə-velt; October 27, 1858 – January 6, 1919), often referred to as Teddy or his initials T. R., was an American politician, statesman, conservationist, naturalist, historian, and writer who served as the 26th president of the United States from 1901 to 1909. He previously served as the 25th vice president under William McKinley from March to September 1901, and as the 33rd governor of New York from 1899 to 1900. Having assumed the presidency after McKinley's assassination, Roosevelt emerged as a leader of the Republican Party and became a driving force for anti-trust and Progressive policies.

Roosevelt was a sickly child with debilitating asthma but partly overcame his health problems by embracing a strenuous lifestyle. He integrated his exuberant personality, a vast range of interests and achievements into a "cowboy" persona defined by robust masculinity. He was home-schooled and began a lifelong naturalist avocation before attending Harvard. His book The Naval War of 1812 (1882) established his reputation as a learned historian and popular writer. Upon entering politics, he became the leader of the reform faction of Republicans in New York's state legislature. His wife and mother both died in the same night and he was psychologically devastated. He recuperated by buying and operating a cattle ranch in the Dakotas. He served as Assistant Secretary of the Navy under President William McKinley and in 1898 helped plan the highly successful naval war against Spain. He resigned to help form and lead the Rough Riders, a unit that fought the Spanish army in Cuba to great publicity. Returning a war hero, he was elected governor of New York in 1898. The New York state party leadership disliked his ambitious agenda and convinced McKinley to make Roosevelt his running mate in the 1900 election. Roosevelt campaigned vigorously, and the McKinley–Roosevelt ticket won a landslide victory based on a platform of victory, peace and prosperity.

Roosevelt assumed the presidency at age 42 after McKinley was assassinated in September 1901. He remains the youngest person to become president of the United States. Roosevelt was a leader of the progressive movement and championed his "Square Deal" domestic policies, promising the average citizen fairness, breaking of trusts, regulation of railroads, and pure food and drugs. He prioritized conservation and established national parks, forests, and monuments intended to preserve the nation's natural resources. In foreign policy, he focused on Central America where he began construction of the Panama Canal. He expanded the Navy and sent the Great White Fleet on a world tour to project American naval power. His successful efforts to broker the end of the Russo-Japanese War won him the 1906 Nobel Peace Prize. Roosevelt was elected to a full term in 1904 and continued to promote progressive policies. He groomed his close friend William Howard Taft to succeed him in the 1908 presidential election.

Roosevelt grew frustrated with Taft's brand of conservatism and belatedly tried to win the 1912 Republican nomination for president. He failed, walked out, and founded the Progressive Party. He ran in the 1912 presidential election and the split allowed the Democratic nominee Woodrow Wilson to win the election. Following the defeat, Roosevelt led a two-year expedition to the Amazon basin where he nearly died of tropical disease. During World War I, he criticized Wilson for keeping the country out of the war; his offer to lead volunteers to France was rejected. He considered running for president again in 1920, but his health continued to deteriorate. He died in 1919. He is generally ranked in polls of historians and political scientists as one of the five best presidents.