It happened on Friday 20th January in the US capital city, Washington DC.
Newsround has everything you need to know about this important moment.
You can start by watching our special video....
The inauguration is the name given to the ceremony where the new president officially starts the job.
At the ceremony, Donald Trump will make a promise to the American people to do his best to protect the country and do a good job of being their leader.
This is called "taking the oath of office".
The official way of doing this is to say: "I do solemnly swear (or affirm) that I will 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."
He will also give a special speech called the "inaugural address", where he will tell everyone some of his plans for when he is leader.
The ceremony traditionally takes place in front of a building called the Capitol.
The Capitol is a building in Washington where part of the US government, called Congress, is based.
Donald Trump will start the day at the White House with the current president Barack Obama, before both of them travel to the Capitol building for the inauguration ceremony.
The inauguration will take place on Friday 20 January, from around 4:30pm.
The UK is 5 hours ahead of Washington, so actually the time will be around lunchtime in America when it takes place.
Many people are expected to go to watch.
People who used to work for the US government will go along and there are also likely to be celebrity guests.
VIPs will get a special spot to watch the ceremony from, but there will also be lots of members of the public who will stand along the National Mall to watch Donald Trump become their next leader.
Last month, he said on social media that he wanted loads of people to come along so they could set a record for the amount of people attending.
Donald Trump has officially become the 45th President of the United States at a ceremony called the inauguration.