Viktor, right, and Amalija Knavs arrive to take the oath of citizenship with their attorney Michael Wildes in New York, Thursday, August 9.
Melania Trump's parents were sworn in as USA citizens on Thursday, completing a legal path to citizenship that their son-in-law has suggested eliminating.
The couple, who had been living in the U.S. on green cards sponsored by Mrs Trump, took the citizenship oath in NY.
They came and went from the ceremony at a Manhattan federal building flanked by Homeland Security police. The average time to process an application for naturalisation in New York City is 11 to 21 months, according to the US Citizenship and Immigration Services (USCIS).
There are only a handful of ways that immigrants to the United States can obtain green cards, and the largest share of them each year are given out based on familial connections. He confirmed that the first lady sponsored their green cards.
Trump's plan, which Congress has resisted, would limit immigrants like his wife to sponsoring only their spouses and underage children to join them in the US, not their parents, adult children or siblings.
Mr Knavs is 74, two years older than President Donald Trump.More news: Asian markets mixed as trade spat in focus, oil stabilizes
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While the Knavs now live in Mar-a-Lago and Trump Tower in New York City, they didn't always live in luxury.
The couple's lawyer said they had "travailed a wonderful journey".
News of the ceremony prompted an immediate response on Twitter, with tweets ranging from "welcome!" to "unfreakingbelievable".
Melania Trump became a United States citizen after entering the country on an Einstein visa for "individuals of extraordinary ability" in 2001 as a model.
The Knavses raised Melania, born Melanija, in the rural industrial town of Sevnica while Slovenia was under Communist rule.
He went on: "It seems like we now have two immigration systems".
Although Melania started from humble roots, she launched a successful modeling career prior to her marriage to the president. "We saw an example of that today".