While the Knavs now live in Mar-a-Lago and Trump Tower in New York City, they didn't always live in luxury.
The Knavs' immigration attorney accompanied the couple to the ceremony.
Michael Wildes, the attorney for the first lady's parents Viktor and Amalija Knavs, told The New York Times on Thursday that their daughter had sponsored them for a green card, and that they applied for citizenship once they were eligible.
He did not specify how long it had taken the Knavs to complete the citizenship process, nor whether the 48-year-old First Lady had sponsored their permanent residency.
The first lady's office declined to comment. First lady Melania Trump's parents have been sworn in as USA citizens.
The family lawyer confirms the Slovenian couple took the citizenship oath in New York City.
It's unclear when the Knavses obtained permanent residency in the US, the Times reports, but Wildes said they met the five year requirement.
Currently, the majority of green card recipients receive them via family sponsorship.
Melania Trump became a USA citizen in 2006 after marrying Trump the year before. "I've been a very staunch advocate against the president's policies myself as an immigration lawyer", Wildes said. A smaller number go to immigrants based on their employment, and other categories include refugees and other special cases.
The Knavses raised Mrs Trump in the rural industrial town of Sevnica while Slovenia was under Communist rule.
She attended high school in the Slovenian capital, Ljubljana, and changed her name to Melania Knauss when she started modelling.