In a special ceremony called an inauguration, President Trump took over from Barack Obama as America's leader.
He won the election in November 2016 but there is a delay of two months as power changes from one President's team to the next.
Mr Trump swore an oath to "faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
In his first speech as President, Mr Trump promised to change America, saying that too much power was in the hands of the government and he would concentrate on giving a voice to ordinary people.
He also repeated his well-known phrase from the election campaign, by promising to "Make America great again".
Donald Trump is the 45th President of the United States of America.