More and more writers are declaring him one.
There are so many definitions it is difficult to say whether Trump qualifies.
In America’s political history a populist was someone who worked to help the poor. Democrat William Jennings Bryan ran three times for the presidency, losing each time.
“In his three presidential bids, he promoted Free Silver in 1896, anti-imperialism in 1900, and trust-busting in 1908, calling on Democrats to fight the trusts (big corporations) and big banks, and embrace anti-elitist ideals of republicanism,” said Wikipedia.
He was called “the Great Commoner.”
The one thing Trump does have in common with him is an opposition to the theory of evolution.
After Bryan came Robert M. La Follette.
“Robert M. La Follette was an American Republican and politician who is best known as a proponent of progressivism and a fierce opponent to corporate power. He served as a member of the U.S. House of Representatives, the Governor of Wisconsin and a U.S. Senator from Wisconsin during his career. He also ran for President of the United States in 1924,” according to Wikipedia.
There were many others who sought to fight for the poor.
It would seem that other descriptions might be more appropriate for Trump. His America First makes it clear he is a nationalist, although he has not avoided using the White House to add to his billions.
He got help from Russian President Putin in defeating favorite Hillary Clinton, though she got 3 million more votes than he did.
It cannot be ruled out yet that he arranged to win three states, Michigan, Wisconsin and Pennsylvania, which gave him a victory in the the electoral college. The race in all three, traditionally Democrat states, was extremely close.