He took the Oath of Office before Chief Justice John Roberts of the U.S. Supreme Court at 1am (midnight Jan 20 Eastern Standard Time in the U.S.).
Trump, who won the U.S. Presidential Election on Nov 8 last year, succeeds Democrat Barack Obama who was president for two terms.
The 70-year-old Trump, a Republican and real estate mogul, defeated his Democrat opponent Hillary Clinton in the election, winning 306 electoral votes to surpass the 270-vote threshold.
The historic oath-taking ceremony, broadcast live by international TV stations, was presided over by Roy Blunt, a Republican senator from Missouri and chairman of the Joint Congressional Committee on Inaugural Activities.
Trump is the first U.S. President with no political or military experience prior to taking office.
Michael R. Pence was sworn in as the Vice President of the United States earlier at the same ceremony.