Theodore Roosevelt Jr. (October 27, 1858 – January 6, 1919), also known as Teddy or T. R., was the 26th president of the United States, serving from 1901 to 1909. Roosevelt previously was involved in New York politics, including serving as the state's 33rd governor for two years. He 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 Era policies.

A sickly child with debilitating asthma, Roosevelt overcame health problems through his strenuous life. He was homeschooled and began a lifelong naturalist avocation before attending Harvard University. His book The Naval War of 1812 established his reputation as a historian and popular writer. Roosevelt became the leader of the reform faction of Republicans in the New York State Legislature. His first wife Alice Hathaway Lee Roosevelt and mother Martha Bulloch Roosevelt died on the same night, devastating him psychologically. He recuperated by buying and operating a cattle ranch in the Dakotas. Roosevelt served as the assistant secretary of the Navy under McKinley, and in 1898 helped plan the 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, Roosevelt was elected New York's governor in 1898. Because the New York state party leadership disliked his ambitious state agenda, they convinced McKinley to choose him as his running mate in the 1900 presidential election. The McKinley–Roosevelt ticket won a landslide victory.

Roosevelt began his presidency at age 42 when McKinley was assassinated. He thus became (and remains) the youngest person to assume the position. As a leader of the progressive movement, he championed his "Square Deal" domestic policies, which called for fairness for all citizens, breaking bad trusts, regulating railroads, and pure food and drugs. Roosevelt prioritized conservation and established national parks, forests, and monuments to preserve U.S. natural resources. In foreign policy, he focused on Central America, beginning construction of the Panama Canal. Roosevelt expanded the Navy and sent the Great White Fleet on a world tour to project naval power. His successful efforts to end the Russo-Japanese War won him the 1906 Nobel Peace Prize, the first American and non-European to win a Nobel Prize. Roosevelt was elected to a full term in 1904 and convinced William Howard Taft to succeed him in 1908.

Roosevelt grew frustrated with Taft's brand of conservatism yet failed to win the 1912 Republican presidential nomination. He founded the Bull Moose Party and ran in 1912; the split allowed the Democrat Woodrow Wilson to win. Roosevelt led a four-month expedition to the Amazon basin, where he nearly died of tropical disease. During World War I, he criticized Wilson for keeping the U.S. out; his offer to lead volunteers to France was rejected. Roosevelt's health deteriorated and he died in 1919. Polls of historians and political scientists rank him as one of the greatest American presidents.