The visit has been presented as a diplomatic coup in Britain, where the idea of the "special relationship" still holds strong, and is being closely watched in Washington as Trump's foreign policy takes shape.
Trump has offered rare support for Brexit among global leaders, seeing parallels with his own election campaign, and saying Britain was "smart" to vote to leave an EU that he believes is now falling apart.
"I'm pleased that I'm able to meet President Trump so early in his administration. That is a sign of the strength of the special relationship," May told parliament on Wednesday.
But analysts note that Trump's protectionist "America First" rhetoric appears at odds with May's promise to turn Britain into a free trade champion.
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"There is a national sympathy from Trump to this country because of Brexit," said Stephen Burman, professor of American politics at the University of Sussex.
"That's not going to change because of a trip".
There is also the uncomfortable issue of Trump's comments about women, some of which he has apologised for.
"Whenever there is something that I find unacceptable, I will say that to Donald Trump," May, Britain's second female prime minister, following Margaret Thatcher, told the BBC.
Asked about the issue in parliament, she said: "I am not afraid to speak frankly to a president of the United States".
May flies to the United States on Thursday, meeting Republican leaders in Philadelphia before heading to Washington for talks at the White House on Friday.
On her trip to Washington, May has said she will highlight her support for free trade, but also the concerns of those who feel they have lost out -- a factor in the votes for Brexit and for Trump.
May told the BBC this week that she was hoping for "early" talks on a US-UK trade deal -- although the European Commission pointed out that Britain cannot start formal trade negotiations until it leaves the bloc.