For his aggressive use of the 1890 Sherman Antitrust Act compared to his predecessors, Roosevelt became mythologized as the "trust-buster". He brought 44 antitrust suits, breaking up the Northern Securities Company, the largest railroad monopoly, and regulating Standard Oil, the largest oil and refinery company. In comparison, Presidents Benjamin Harrison, Grover Cleveland, and William McKinley combined prosecuted only 18 antitrust violations under the Sherman Antitrust Act.

In 1902, Roosevelt was invited on a bear-hunting trip by Mississippi Governor Andrew H. Longino. After several of the other hunters had already killed an animal, one of Roosevelt's assistants cornered an older black bear with dogs and tied it to a tree for Roosevelt to shoot. The president refused, seeing it as terribly unsportsmanlike. The story inspired candy store owner Morris Michtom to create a stuffed black bear which he called a "teddy bear" and mass-produced with the president's blessing.

The United States Department of Commerce and Labor was concerned with controlling the excesses of big business. The department was renamed the Department of Commerce on March 4, 1913, and its bureaus and agencies specializing in labor were transferred to the newly created Department of Labor.

Theodore Roosevelt easily defeated the Democratic nominee, Alton B. Parker, winning both the popular vote (56%) and the Electoral College (336 to 140). Roosevelt's victory made him the first president to win a term in his own right after having ascended to the presidency upon the death of his predecessor.

As vice president, Charles W. Fairbanks worked against Roosevelt's progressive policies. Fairbanks unsuccessfully sought the Republican nomination at the 1908 Republican National Convention and backed William Howard Taft in 1912 against Roosevelt.

When Russia went to war with Japan in 1904, Roosevelt offered his services as an arbitrator. After initial resistance, both sides came to the bargaining table in New Hampshire in 1905, where Roosevelt brokered the peace settlement that won him the Nobel Peace Prize.

Though conservatives initially opposed the bills, Upton Sinclair's The Jungle, which fictionalized the horrors of the meatpacking industry, including scenes of immigrant workers falling into boiling vats of lard, helped galvanize support for reform.

Roosevelt had a lifelong interest in pursuing what he called "The Strenuous Life". To this end, he exercised regularly and took up boxing, tennis, hiking, rowing, polo, and horseback riding. As governor of New York, he boxed with sparring partners several times each week, a practice he regularly continued as president until being hit so hard that he suffered a detached retina and became blind in his left eye (a fact not made public until many years later).

Of all Roosevelt's achievements, he was proudest of his work in conservation of natural resources and extending federal protection to land and wildlife. He established the United States Forest Service and signed into law the creation of five National Parks, 51 bird reserves, four game preserves, and 150 National Forests. The area of the United States that he placed under public protection totals approximately 230 million acres (930,000 square kilometers).

Theodore Roosevelt and family had many pets in the White House, including snakes, guinea pigs, hens, ponies, rabbits, dogs, cats, rats, lizards, a barn owl, a black bear, a badger, a one-legged rooster, and a laughing hyena named Bill.