He said the pair had been living in the United States on green cards sponsored by Mrs Trump.
A lawyer for Viktor and Amalija Knavs said the Slovenian couple took the citizenship oath on Thursday in New York City. The first lady herself was granted citizenship in 2006. The Slovenian immigrants, a former vehicle dealer and textile factory worker, had been living in the U.S.as permanent residents.
The idea that family-based immigration could be labeled as "dirty" might have a lot to do with the Knavs' son-in-law.
A source familiar with the situation told Yahoo News the Knavses received their green cards before Trump was elected and that they were sponsored by their daughter. "I've been a very staunch advocate against the president's policies myself as an immigration lawyer", Wildes said. Green cards can also be obtained through employment, receiving asylum or refugee status, or in other special circumstances.
The president in January proposed ending most family-based immigration and replacing it with a skills-based system.
Mr Trump and his congressional allies have fought to slash that dramatically, limiting sponsorship to spouses and minor children, including dropping the threshold for minor children from 21 to 18.
It's unclear whether Mr and Mrs Knavs had their green cards granted by a process the President Trump has sought to end.
The Knavses raised Melania in the rural industrial town of Sevnica while Slovenia was under Communist rule.
President Donald Trump's parents-in-law have become U.S. citizens in a private ceremony.
Mr Knavs, who at 74, is two years older than Mr Trump, is thought to have worked as a auto dealer while Mrs Knavs, 73, worked in a textile factory.
She settled in NY in 1996, met the future president two years later and became his third wife in 2005.