A French scientist was prevented from entering the United States this month because of an opinion he expressed about the Trump administration’s policies on academic research, according to the French government.

Philippe Baptiste, France’s minister for higher education and research, described the move as worrying.

“Freedom of opinion, free research and academic freedom are values we will continue to proudly uphold,” Mr. Baptiste said in a statement. “I will defend the possibility for all French researchers to be faithful to them, in compliance with the law, wherever they may be in the world.”

Mr. Baptiste did not identify the scientist who was turned away but said that the academic was working for France’s publicly funded National Center for Scientific Research and had been traveling to a conference near Houston when border officials stopped him.

The U.S. authorities denied entry to the scientist and then deported him because his phone contained message exchanges with colleagues and friends in which he expressed his “personal opinion” on the Trump administration’s science policies, Mr. Baptiste said.

It was not immediately clear what led the border authorities to stop the scientist, why they examined the contents of his phone or what they found objectionable about the conversations.

Customs officers are allowed to search the cellphone, computer, camera or any other electronic device of any travelers crossing the border, according to U.S. Customs and Border Protection, although the agency says that such instances are rare. In 2024, less than 0.01 percent of arriving international travelers had their electronic devices searched, according to the agency.

Mr. Baptiste’s office declined to provide further details about the case. A spokesman for the American Embassy in Paris also declined to comment.

A spokeswoman for the National Center for Scientific Research said that the scientist who was turned back did not wish to speak to the media and declined to comment further.

The Agence France-Presse news agency reported earlier on the scientist’s refused entry to the United States.

Mr. Baptiste, the minister, has been particularly vocal over the past few weeks in denouncing threats to academic freedom in the United States, where funding cuts and layoffs by the Trump administration have targeted institutions of higher education, scientific research and the federal government’s own scientific work force.

Mr. Baptiste has urged French universities and research institutes to welcome researchers seeking to leave the United States.

“Europe must be there to protect research and welcome the talent that can contribute to its success,” Mr. Baptiste wrote on social media after meeting with his European counterparts in Warsaw on Wednesday to address “threats to free research in the United States.”

Jennifer Jones, the director of the Center for Science and Democracy at the Union of Concerned Scientists, an advocacy group in the United States, said she worried that cases like the one involving the French scientist would have a chilling effect on research collaboration across borders.

“My fear is that these are early cases with many more to follow,” Dr. Jones said. “I am hearing from my network that people are very concerned about any kind of international travel in either direction.”

“And that should worry all of us,” she added. If scientists limit their movements to conferences and other events designed to advance research, she said, “it is the public that is going to suffer.”

Ségolène Le Stradic contributed reporting.