First lady Melania Trump's parents, Viktor and Amalija Knavs, were sworn in as United States citizens on Thursday.
Viktor and Amalija Knavs took their oaths in NY, the Associated Press reports.
Melania Trump became a citizen in 2006 after obtaining a green card.
He said: "It's a dirty - a dirtier word", but added: "It stands for a bedrock of our immigration process when it comes to family reunification".
In 1996, she moved to New York City where she worked as a model and met Donald Trump two years later. Trump has advocated a "merit-based" system, but has not proposed any method of admitting immigrants to the USA to replace those categories. "This is an example of it going right", he said.
A source familiar with the situation told Yahoo News that the Knavses received their green cards before Trump was elected and that they were sponsored by their daughter.
Currently, the majority of green card recipients receive them via family sponsorship.
There are only a handful of ways that immigrants to the U.S. can obtain green cards, and the largest share of them each year are given out based on familial connections.
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He maintains there should instead be a merit-based system, which prioritises professionals over relatives, and has drawn censure for his strident attacks on immigration laws and immigrants.
They came and went from the ceremony at a federal building flanked by Homeland Security police.
She settled in NY in 1996 and met Mr Trump two years later.
In November 2016, the AP reported that she was paid for several modeling jobs in the USA before obtaining a work visa, which would be illegal under immigration law.
Kaitlyn Schallhorn is a Reporter for Fox News.
The Knavses' citizenship ceremony was kept hush-hush at the Jacob K. Javits Federal Building.
Mr Knavs, 74, was a vehicle dealer, while his wife, who is 73, worked in a textile factory. They would appreciate their privacy and we just thank everybody for their attention to this very important dialogue that we are having on immigration.
On Thursday, the president and first lady were vacationing at the Trump National Golf Club in Bedminister, New Jersey, and she did not accompany her parents to the ceremony.