Theodore Roosevelt was born in New York City on October 27, 1858, to parents Theodore Roosevelt, Sr., and Martha Bulloch Roosevelt. He was to be the second oldest of four children, along with an older sister, Anne; a younger sister, Corinne; and a younger brother, Elliott. Mostly out of affection, but partly to distinguish his name from his father's, Theodore's family often referred to him as "Teedie" and continued to call him this name into his late teenage years.

The Roosevelt family had a long, distinguished history in New York City and had earned its way into affluent social circles. More than seven generations of Roosevelts had been born on Manhattan, nearly all of them following the family profession to become businessmen and merchants. In 1644, Teedie's ancestor Klaes Martensen van Roosevelt (also spelled Claes van Rosenvelt) sailed across the Atlantic from Holland to become one of the first settlers in New York City, which was called New Amsterdam at the time. By the 1800s, the Roosevelts operated a hardware and industrial glass firm, Roosevelt & Son, which was eventually run by Teedie's grandfather, Cornelius Van Schaack Roosevelt. This family business contributed significantly to the family's wealth. Teedie's father, Theodore Sr., eventually inherited control of the family firm along with Teedie's uncle, James Roosevelt. Most of the Roosevelt wealth, however, came from land holdings in and around New York City.

Teedie's mother came from a similarly prominent family in the South. Originally from Scotland, the Bulloch family first immigrated to South Carolina, but eventually settled in Georgia to become plantation owners. Theodore's great- grandfather, Archibald Bulloch, served as the first Revolutionary President of Georgia, while many more Bullochs fought in the American Revolution against Great Britain. A slave-owning family, the Bullochs sent their sons to fight for the Confederacy during the Civil War. Martha's brother and Teedie's uncle, James Bulloch, served as an admiral in the Confederate Navy and was responsible for bringing the infamous warship Alabama from England. James's brother, Irvine Bulloch, also served in the Confederate Navy as a midshipman on the same warship. Although Teedie's family favored opposites sides during the war–Theodore Sr. and the children supported the Union while the Bulloch women supported the cause of the Confederacy–the war did not cause any serious hardships for the family.

Teedie was a weak and sickly child for many years. He suffered from severe asthma, a seriously debilitating condition that was not entirely understood at the time. To combat the coughing and respiratory problems, doctors prescribed everything from vacations on the coast to smoking cigars to drinking coffee and whiskey. Teedie also suffered from headaches, toothaches, and abdominal pains. The Roosevelts tried every suggested treatment but none worked. As a result, Teedie was often confined indoors and unable to play with other children his age. He was entirely privately tutored until he entered college. His confinement indoors was probably the reason he began writing his thoughts, feelings, and experiences in journals, a practice he continued until the night of his death. Teedie also became much closer to his father because of his asthma; Theodore Sr. spent many sleepless nights with his son, reading to him, talking with him, and taking him for midnight carriage rides through the city to distract him from his pains. Teedie idolized his father, considered him to be his best friend, and inherited a strict moral conscience from him. For the rest of his life Roosevelt evaluated everything he experienced through this sense of morality; he saw things in terms of black and white, right and wrong.

While confined indoors, Teedie also discovered the joy of reading and learning. He was interested in all types of books but particularly enjoyed tales of adventure, a love that never diminished. Somewhat ironically in light of his health, Teedie also loved the outdoors. Asthma permitting, he spent as much time as possible outside, observing nature and collecting plants and animals to inspect. His bedroom in the Roosevelt family mansion often smelled of preserving chemicals and dead animal specimens he planned to stuff. When Teedie was twelve, Theodore Sr. lectured him on the importance of developing the body in order to fully develop the mind. According to legend, Teedie was taunted by a couple older and stronger boys several months after this lecture, and afterwards determined to build his strength so that he would never feel such shame again. Theodore Sr. built Teedie a small gymnasium in the house for him to improve his strength. The boy also began boxing lessons. Eventually Teedie's asthma disappeared and his strength increased.

When Teedie was ten and fourteen, the Roosevelt family took two extensive vacations throughout Europe and the Middle East. The trips took them through England, Holland, France, Prussia (Germany), Italy, Switzerland, Austria, Hungry, Turkey, Greece, Egypt, and modern-day Syria, Lebanon, and Israel. Teedie was particularly fascinated with Egypt. While on a trip up the Nile River, he marveled at Alexandria, Cairo, Thebes, the Pyramids of Giza, and the various tombs and temples along the way. He also used this opportunity to collect various species of plant life and to hunt birds and small animals to add to his budding natural history collection. Although later on Roosevelt claimed he was much too young to fully appreciate his first trip to Europe, he never forgot the sites hew saw and the general spirit of adventure he gleaned from his second trip as a teenager.

In 1876, when he was eighteen, young Theodore entered Harvard College. Like many freshmen, he found his first year rather rocky, but by the second year had found his place and was earning honors grades in his courses. Later in life he would write that he "thoroughly enjoyed" his time at Harvard. He originally intended to become a scientist, but realized later that the scientist's life was not for him. Although he continued to take science and natural history courses, he also concentrated on history, philosophy, political economy, and German.

Theodore was quite popular with the other men on campus. He was invited to join the prestigious Porcellian and Hasty Pudding Clubs, and became president of Alpha Delta Phi, vice-president of the Natural History Society, joined the glee club, was a member of the Class Committee, and edited The Harvard Advocate, an undergraduate publication. He was also a member of Phi Beta Kappa, taught Sunday school every week, and boxed for the Harvard boxing team. His classmates began calling him "Teddy," and the name stuck. One of his classmates, Robert Bacon, later became his Secretary of State during the end of his presidency. Teddy graduated from Harvard in 1880.