A lawyer for Viktor and Amalija Knavs revealed the Slovenian couple, who had already been living in the United States as permanent residents, took the citizenship oath in New York City.
The first lady's office declined to comment.
Trump hasn't commented or tweeted about his in-laws' newfound citizenship.
It's unclear when they obtained their green cards, but usually, permanent residents have to hold green cards for five years before applying for citizenship.
When the Times asked Wildes if the Knavs obtained their citizenship through family-based immigration ― often pejoratively referred to as "chain migration" ― Wildes was vague. A smaller number go to immigrants based on their employment, and other categories include refugees and other special cases.
Trump has taken a hardline on immigration policy, criticizing so-called main migration that allows naturalized U.S. citizens to sponsor close relatives for permanent residency.
Advocates for restricting legal immigration have pointed to the imbalance in favour of family connections as evidence of the need for reform, calling for a "merit-based" system that would choose immigrants based on need in the US.
Viktor Knavs is 74, two years older than his son-in-law.
The Knavses are from Slovenia, but now divide their time between New York City, Palm Beach, and Washington, D.C., where they stay with the Trumps in the White House.
The first lady, born Melanija, changed her name to Melania Knauss when she started modelling.