US President Donald Trump on Friday signed into law a sweeping overhaul of the US tax code, two days after the Republican plan was adopted by Congress in a major political victory.

Trump signed the bill shortly before departing for his Mar-a-Lago resort in Florida, pulling forward a formal signing initially planned for January.

"I watched the news this morning and they were all saying, will he keep his promise, will he sign it by Christmas?" Trump told reporters gathered in the Oval Office.

"And I called downstairs and said 'Get it ready, we have to sign it now.'"